Parasites, bacteria, and viruses have been the scourge of humanity as long as we have been here, but disease has reshaped our history and influenced our evolution. Parasites helped give our immune systems the boost it needed to get up and running, and the humble bacterium has helped dictate the form this planet has taken. Sometimes, it seems that we humans are simply playthings in their hands, but they haven’t just been capricious forces that toss us around like rag dolls. These microorganisms have also done incredible things to help humanity.

10The Viruses We Carried Out Of Africa Helped Us Survive

Thanks to the science of viral molecular genetics, we now know quite a bit about the bugs that infected us along our evolutionary path, and we have found that these hitchhikers have done quite a bit to help us along the way. For example, it was the evolutionary pressure they placed upon our immune system that made it as robust as it is today. Additionally, viruses may have played a role in the loss of specific receptors that we once possessed on the surface of our cells that infectious agents could latch onto and use to cause disease. By ridding the human body of this source of disease, viruses created a safer environment for themselves, benefiting everybody involved.

But they may have also played a role in ensuring that, among competing hominid species, it was Homo sapiens that came out on top. While our species was developing, disease and parasites encouraged genetic diversity and weeded out the unfit. Once the first Homo sapiens left the continent, they brought their infectious agencies and parasites with them. If you’ve read about North American and European smallpox, you know how this goes.

While it wouldn’t have been the only factor, viral parasites would spread to other hominids like Homo neanderthalensis (Neanderthals), who wouldn’t have had any previous exposure to the new bugs and possessed a nasal structure that was less efficient at filtering air and keeping new viruses at bay. They would have devastated other hominid species, because the bugs were primed to live in similar environments, but the hominids were not primed to receive them. Models have shown that if Neanderthals had a mortality rate only 2 percent higher than humans, it would have been sufficient to cause their extinction after 1,000 years of competition. While disease was doubtless not the only factor, it would have certainly played a large role.

Most models of human disease evolution claim that they mainly evolved during the Neolithic era, after man moved out of Africa and populations increased, so there is some evidence of this selective viral pressure. Many of these early viruses have even been so successful that their genes have literally become a part of our DNA. For example, the human genome has been found to contain genes from the borna virus that were gained about 40 million years ago. In fact, scientists have isolated about 100,000 elements of human DNA that have come from viruses, mostly within what is called our “junk DNA.” The viruses that make up the majority of our junk DNA are called endogenous retroviruses, and they are so much a part of us that a scientist recently brought one “back to life” and even infected hamsters and cats with it.

9Modern-Day Medical Uses Of Leeches And Maggots

For thousands of years, the European leech (Hirudo medicinalis) was used in medicine for bloodletting purposes, treating a wide range of disorders from hemorrhoids to ear infections. The practice goes so far back in time that an Egyptian painting from 1500 B.C. depicts their use. While some nations have never stopped using them, the practice fell out of favor in the Western world with the knowledge of bacteria and subsequent focus on the germ theory for medical treatment.

In the 1970s and 1980s, though, leeches made a comeback. Cosmetic and reconstructive surgeons found that they were an effective method for draining blood from swollen faces, black eyes, limbs, and digits. They are also helpful for reattaching small body parts like ears and flaps of skin, because they draw away blood that could clot and interrupt the healing process. Leeches have saved people from amputations and may even relieve the pain of osteoarthritis. Even veterinarians sometimes use them.

Maggots, on the other hand, are nature’s clean-up crew. They’re great for eating away dead or infected flesh, revealing the healthy tissue below in a process called debridement. They have also been found to be an effective treatment for ulcers, gangrene, skin cancer, and burns, among other things.

Maggots and leeches, as gross as they may be, are so effective that the FDA classified them as the first “live medical items” in 2010, paving the way for an entire industry called biotherapy. An organization called Biotherapeutics Education and Research Foundation (BTERF) has even sprung up to raise awareness of the new uses for these old critters, and there are several companies that sell them.

8Parasites And Our Immune Systems May Have Co-Evolved To Protect Us From Allergies

Researchers studying the effects of gastrointestinal parasites have come up with an astonishing theory: After parasites first colonized our gastrointestinal systems, they evolved over millions of years the ability to suppress our immune systems. At the same time, our own bodies evolved to partially compensate for the effect.

The astonishing part, and what this means for human health, is that once parasites and harmless microorganisms present in water and soil have been largely removed from their natural environment inside of us in developed nations through the use of modern medicine, our immune systems actually overcompensate for their loss, leading to allergies and even increased chances for asthma and eczema.

This “old friends” hypothesis (sometimes referred to as the “hygiene hypothesis,” though it’s actually more of a complementary theory) has gained more support in recent years as we identify new ways microorganisms have helped us survive over the eons. Clinical trials have been conducted using worms to test against multiple sclerosis, IBD, and allergies.

The main proponent of the old friends hypothesis is Graham A.W. Rook of University College London. He first proposed it in 2003, and since then, it has also been proposed as a possible cause of some forms of stress and depression.

Some people have taken the old friends hypothesis to its ultimate logical conclusion that if removing our parasites from society has led to health problems, we should put them back. In 2008, University of Wisconsin professor of neurology John Fleming conducted a clinical study in which he infected multiple sclerosis patients with parasitic worms to test their effectiveness against the disease. Over a period of three months, patients who had an average of 6.6 active lesions around their brain’s nerve cells were reduced to an average of two. When the trial was over, the number of lesions shot back up to 5.8 within two months. In earlier trials, the parasites appeared to have positive effects upon ulcerative colitis and Crohn’s disease as well.

Parasite therapy is still in the experimental phases, however, and probably has negative effects that outweigh the positive ones. As of now, the FDA has classified the worms as biological products that cannot be sold until proven safe. Only one species, Trichuris suis, has been approved for testing under Investigational New Drug (IND) status.

7Virotherapy

One of the most exciting and promising branches of medicine in recent decades is virotherapy, a biotechnology technique to reprogram viruses to treat disease. In 2005, researchers at UCLA announced that they had turned one of humanity’s deadliest enemies into a cancer-killer when they reprogrammed a modified strain of HIV to hunt down and destroy cancer cells. Around the same time, researchers at the Mayo Clinic in Rochester, Minnesota modified the measles virus to do the same.

The technique is similar to the one used to breed genetically engineered plants, in that a virus is used as a gene-delivery vehicle. It has long been recognized as the most efficient means of gene transfer. This system is used for the production of useful proteins in gene therapy and has great potential for the treatment of immunological disorders such as hepatitis and HIV.

Viruses have been known to have the potential to treat cancer since the 1950s, but the advent of chemotherapy slowed its progress. Today, virotherapy is proving to be extremely effective against tumors without harming the healthy cells around it. Clinical trials of oncolytic virotheraphy have shown low toxicity and promising signs of efficacy. In 2013, a drug called talimogene laherparepvec (TVEC) became the first drug based on a tumor-killing virus to succeed in late-stage testing.

One of the biggest challenges facing researchers is how to deliver the virus where it will do the most good before the body recognizes it as an intruder and mounts a defense. Current research is looking into finding natural tumor-targeting “carriers,” cells that can deliver the virus without either the cell or the virus losing its normal biological functions.

6Using Viruses To Cure Bacterial Infections

Bacteriophages are viruses that specifically attack bacteria. First recognized by Frederick Twort in 1915 and Felix d’Herelle two years later, they have been used to study many aspects of viruses since the 1930s. They are especially common in soil, where many species of bacteria make their home.

Because phages disrupt the metabolism of bacteria and destroy them, it has been long recognized that they could play a role in treating a wide range of bacterial diseases. Because of the innovation of antiobiotics, though, phage therapy was mostly shelved until the rise of antibiotic-resistant bacteria generated a renewed interest in the field.

An individual phage species is generally only effective against a small range of bacteria or even one specific species (its primary host species), which was originally seen as a disadvantage. As we have learned more about the beneficial aspects of our natural flora, though, it has come to be recognized as the advantage that it is. Unlike antibiotics, which tend to kill bacteria indiscriminately, bacteriophages can attack the disease-causing organisms without harming any other bacteria living inside us.

While bacteria can develop resistance to both antibiotics and phages, it only takes a few weeks rather than a few years to develop new strains of phages. Phages can also have an easier time penetrating the body and locating their target, and once the target bacterium is destroyed, they stop reproducing and soon die out.

5Vaccines

Beginning in the 1790s, when Edward Jenner developed the world’s first vaccine against smallpox using a less virulent strain called cowpox to inoculate patients, vaccines have saved countless millions of lives. Since then, several different types of vaccines have been developed. Attenuated or “live” vaccines use live viruses that have been weakened or altered so that they do not cause illness, while inactivated or “killed” vaccines contain dead microorganisms or toxins that are usually used against bacterial infections. Some vaccines—including subunit and conjugate vaccines, as well as recombinant and genetically engineered vaccines—only use a segment of the infectious agent.

When a vaccine is injected, the pathogen goes to work, but there is not enough of it to replicate at the rate it needs to in order to take hold. The body mounts an immune response, killing the pathogen or breaking down the toxin responsible for disease. The body’s immune system now knows how to fight the disease and will “remember” if it comes across it again. In other words, scientists have figured out how to get a pathogen to help its own target defend itself against it. They have even taken the first steps toward developing vaccines for several forms of cancer, with three vaccines approved by the FDA for the hepatitis B virus (which causes liver cancer), human papillomavirus types 16 and 18 (which cause cervical cancers), and metastatic prostate cancer in some men.

Thanks to vaccines, several diseases have been driven to virtual extinction. Smallpox is the most famous example, but polio, though not totally eradicated, comes in at a close second. Several other diseases might be gone by now if vaccines weren’t so hard to come by in the underdeveloped nations that still struggle with them. Things are getting worse instead of better, with diseases coming in from an unexpected source: affluent, educated Westerners who should know better.

Unfortunately, the anti-vaccination movement is making a comeback in regions where these diseases were once under control. Before the introduction of the measles vaccine in 1963, approximately 500,000 people per year were infected in the US, 500 of whom—mostly children—ended up dead. By 1983, there were only 1,497 cases reported, and after a brief resurgence in the ’80s and ’90s, reported cases were down to just 37 in 2004. After the anti-vaccination movement began gaining traction, 118 cases were reported in the US alone in 2011. That number keeps growing, fed by travelers coming in from areas with higher rates and finding less resistance. Whooping cough, once thought to be gone forever in the US, is also on the rise.

4Bacterial Waste Breakdown

Some of the smallest and simplest of creatures on Earth play some of the most important roles in safeguarding all of life. Bacteria have perhaps the most important role of all: breaking down and recycling waste.

The dead remains of animals and plants, along with the excrement of all organisms, contain vital nutrients and stored energy. Without a way to reclaim these nutrients, though, the available sources would be quickly depleted. Luckily, many bacterial species feed upon these energy sources, breaking them down to their smallest molecules and returning them to the soil, where they reenter the food chain.

As helpful as this process already is, humans have found many ways to exploit it for a variety of even more advantages. Bacteria are used in sewage treatment, industrial waste management, and the clean-up of oil spills, leaked pharmaceuticals, and wastewater. They have also been useful in the development of aqua-farming, algae control, and waterless toilets. Researchers and engineers are currently looking into their potential use in the production of environmentally friendly bioplastics, glues, and building materials. They may even be used to break down plastic waste.

3We Would Quickly Die Without Our Gut Bacteria

Poorly understood until recently (and there is still quite a bit of research to be done), the natural bacteria that lives in our guts works with our immune system to drive out pathogens, produce vitamin K, stimulate peristalsis, and perhaps most importantly, digest our food. Without our gut bacteria, we wouldn’t be able to perform any of these functions, and we would quickly die.

The more we learn about beneficial strains of gut bacteria, the more we can incorporate that knowledge into healthy living. After it was determined that certain gut bacteria can play a role in obesity, probiotics became all the rage. Probiotics are the bacteria that reside in fermented foods and are now sold as supplements. Bacteria like some species of bifidobacteria, found in most yogurts, can create a highly acidic environment in which less-beneficial microorganisms cannot survive. Fatty foods and stress can also play a role in the health of our stomach flora, killing beneficial bacteria while favoring the more harmful kind that cause gas, bloating, and “leaky gut syndrome.”

In a huge breakthrough in the study of our gut bacteria and what they do, a team of Chinese and Danish researchers have recently developed a new way to identify these microorganisms using DNA sequence data. They identified over 500 species of benign bacteria and 800 new species of viruses that could live off them, providing hope for new ways to treat diseases associated with them, such as diabetes, obesity, and asthma.

2Skin Bacteria Serve As Our First Line Of Immune System Defense

The moment you emerged from your mother’s womb, you were set upon. They ambushed you in mere moments and colonized every inch of your skin, and they have been with you ever since. They are prokaryotes and other bacteria, and without the evolutionary partnership humans forged with them millions of years ago, you would have been dead soon after being born.

One of the most common skin bacteria is Staphlococcus epidermis, a bug that we now know plays a role in fighting off Leishmania major, the cause of a nasty disease called leishmaniasis that results in skin boils and open sores that don’t heal. The good bug triggers an immune response called IL-1 that the body can’t produce on its own, making Staphlococcus a necessary part of the human body, as vital to our existence as any organ.

Prokaryotes, which also colonize the digestive tract, cover every exterior surface on the skin. Along with the rest of our beneficial skin microbiota, they became a part of us when they started competing against less-benevolent microorganisms for real estate. Along with the immune cells in our skin, they protect us against both pathogenic bacteria and opportunistic fungi that try to invade. This allows our bodies to spend less energy defending our exteriors and focus more on things like fighting viruses and precancerous cells.

While there is still much to learn before we can really use this knowledge in our health regimens, we are already looking to a future that involves the purposeful use of skin bacteria. A start-up based in Massachusetts called AOBiome, for example, has created a body spray made of live cultured chemoautotrophic bacteria called Nitrosomonas. They claim that their spray can “replenish healthy skin bacteria” and even replace showering, as the bacteria live off the ammonia in our sweat.

1Life As We Know It Wouldn’t Be Here Without Cyanobacteria

Cyanobacteria, or blue-green algae, are possibly the oldest still-living species on Earth, with fossils dating back 3.5 billion years. They are unicellular bacteria that grow in colonies, and if it weren’t for them, you wouldn’t be here, and neither would nearly every other form of life.

Cyanobacteria were the world’s first photosynthesizers. They used energy from the sun along with chemicals in primordial oceans and inert nitrogen in the atmosphere to make their food. As a waste product, they generated oxygen, a poison to virtually every other form of life at that time and the cause of early mass extinction events. Over a period of roughly 300 million years, all this oxygen generation helped form the atmosphere as we know it, during the Archaean and Proterozoic eras.

That wasn’t the only way this bacteria kick-started life as we know it. Sometime during the Proterozoic or early Cambrian era, they formed a symbiotic relationship with certain eukaryote cells, making food for the cell in return for a stable environment to call home. These were the first plants, as well as the origin of eukaryotic mitochondria, which is essential for animal life. This truly titanic event is now known as endosymbiosis.

While several forms of cyanobacteria are toxic, a species named Spirulina was an important food source for the Aztecs and eaten regularly by many Asian nations. Today, it is often sold in powder or tablet form as a health food supplement.

Lance LeClaire is a freelance artist and writer. He writes on subjects ranging from science and skepticism to religious history and issues to unexplained mysteries and historical oddities, among other subjects. You can look him up on Facebook.

You are not going to make it through life without getting sick. It happens to the best of us. What kind of sickness you end up with depends on a number of factors. Some illnesses are far easier to get over than others. Some are pretty much death sentences the moment you’re diagnosed.

The cause of your illness can also vary greatly. Did you eat something that had gone bad? Chances are you picked up some kind of bacteria. Did you get sick after going swimming in some polluted water? You might have got a parasite. Did somebody sneeze on your bagel? You might have a virus. And let’s not forget that, if there’s mold growing in your house, you could be infected with a deadly kind of fungus. What a fun group!

In an ideal world, you’ll just avoid every kind of dangerous bacteria, parasite, fungi, and virus. But the world is rarely ideal. So what’s the most dangerous one of them all? If an evil wizard trapped you in a room with four doors, which would be the one that you’d be most likely to survive a walk-through? Let’s look!

The Basics

In general, a virus is more dangerous than a bacterium. Take that with a grain of salts, of course, because circumstances will vary. The virus that causes a cold is probably not going to be as dangerous to you as botulism.

Bacteria are single cells, and they’re able to survive on their own. Most bacteria are harmless, some are even helpful. Your gut has 100 trillion bacteria in it right now to help you digest your food. Only a small number of them are actually going to cause you any harm. Bacteria can be 10 to 100 times larger than viruses, or about one to three microns in length. Salmonella is a common bacterium.

On the other hand, viruses don’t do well on their own. They have a parasitic nature and require a host to help them survive. They need your body to reproduce and proliferate the cost of which is you getting sick and maybe dying. Viruses may be as small as 20 to 200 nanometers in diameter. 

Parasites, part of a group called eukaryotes (meaning their cells have a nucleus and internal structures) are larger than viruses and often bacteria as well. Some parasites can be whole, living organisms, like a tapeworm. 

Fungi are most often found in the form of spores and molds. Athlete’s foot is a kind of fungal infection.

Bacteria Breakdown

A single bacterium is a single cell. It is one complete little microorganism all on its own and can live outside of a human body. In fact, many bacteria happily reproduce in the soil, in rotten food, on your skin and anywhere else the conditions are right.

The dangerous kinds of bacteria can affect your body in a number of ways.  Many dangerous bacteria are able to produce toxins which can be deadly and are what lead to serious infections in the body. The toxins can paralyze the cells in your body or even destroy them, disrupting normal cell function and causing serious damage. Others can reproduce so prolifically that they crowd out your normal, healthy cells.

Thanks to antibiotics, medicine has been able to save countless lives by treating bacterial infections. Antibiotics can either kill or slow the growth of bacteria. They do this by either destroying the bacteria cell wall or limiting its ability to grow and reproduce.

Because bacteria are able to reproduce very quickly, some every 20 to 30 minutes, they’ve also been able to mutate quickly. This has given rise to antibiotic-resistant bacteria. Over time, the bacteria that has survived has mutated to develop various defense mechanisms against antibiotics. Some may produce enzymes that destroy antibiotics, and others have ways of removing the antibiotic before it can reach its target.

Some more common bacteria, like salmonella, gonorrhea, and campylobacter, have developed strains that are resistant to antibiotics. This means that any infection that may have been considered easily treatable could become far more dangerous and deadly as it evolves. 

Because of the ever-changing nature of bacteria, it’s hard to choose the “worst” of them all. In 2024, the World Health Organization issued a list of dangerous, drug-resistant bacteria and there were 15 chosen. Near the top of the list were bacteria that cause tuberculosis, called mycobacterium tuberculosis. This is potentially the deadliest bacterium in the world and is responsible for 1.7 million deaths per year. 

Virus Breakdown

Viruses are not cells or living organisms on their own. Instead, a virus is a small amount of genetic material that is encased in protein. A virus can only work if it’s inside a living organism. On its own, it’s nothing, it has no function or purpose. 

Once inside a host organism, the virus uses the cells of that living organism to replicate. This process can end up destroying cells and leading to infections. Because viruses are so small, they can even infect bacteria and fungi. You can breathe them in or contract them through things like mosquitoes in ways that bacterial, parasitic, and fungal infections can’t because they are larger.

While a host body will try to produce antibodies to fight off a virus, if they get overwhelmed, and the virus can replicate faster than the host organism can fight it off, that’s when sickness takes over and the potential outcome is death. The viruses need your cells to reproduce because they don’t have the material or energy to do it on their own. This process destroys your cells.

Your body will fight back by raising the temperature. That’s what a fever is. Viruses tend to need a very delicate balance of temperature to survive and even a few degrees too warm can kill them. Unfortunately, a prolonged fever is dangerous for you as well. 

In addition, your immune system will try to use antibodies to fight the virus if it can. It needs to be exposed to a pathogen before it can make antibodies, however. If the virus is something you have never experienced, you will have no antibodies at first and your immune system may not be able to fight back. 

A virus like Ebola is extremely deadly. Up to 90% of people who contract Ebola will die from it. While that’s terrifying, it also ends up being one of Ebola’s weaknesses. Because it kills so quickly, the disease is not able to spread as well as others that don’t have such a high, swift mortality rate. Outbreaks, once isolated, tend to burn themselves out before they spread too far from the source. 

Judging the danger of a particular virus can be tricky. While Ebola kills up to 90% of people who contract it, it’s not widespread. However, HIV has spread all over the globe and is arguably one of the deadliest diseases in history. As many as 32 million people have died from HIV. That said, new treatments have greatly reduced the overall mortality rate, and the odds of dying from HIV now, with treatment, are very low.

In 1918, a flu pandemic killed somewhere between 50 million and 100 million people. 

Rabies is another virus that can seem quite common but is exceedingly dangerous. Without proper treatment, the mortality rate for humans infected with rabies is nearly 100%.

Viruses we have mostly eradicated were far deadlier than what most modern people can understand. Smallpox, for instance, killed about 300 million people. 

Fungi Breakdown

Thanks to pop culture, most people are familiar with just how terrifying a fungal infection can be. What first gained attention as quirky articles on the internet were then made into a worldwide phenomenon in the game and subsequent TV show called The Last of Us.

In The Last Of Us, much of mankind has been wiped out by a fungal cordyceps infection. Cordyceps is a real thing that has been seen infecting far less complex organisms like ants. The fungus grows inside of them, literally breaks through their bodies, and forces them to keep moving like zombies even when they should be dead. Fascinating stuff, but not applicable to humans. Our immune systems are vastly more complex than an insect’s and, as a result, we are immune. Unless it mutates one day. 

While cordyceps won’t kill you anytime soon, it doesn’t mean other fungi aren’t a danger. In 2023, the CDC warned about the rapid spread of Candida Auris. The fungus is resistant to most antifungal medications and was putting people in the hospital on ventilators. The infection can spread into your heart, lungs, blood, eyes, bones and organs.

Another fungus, cryptococcus neoformans, has a mortality rate between 41% and 61% and is especially dangerous to those with an already compromised immune system. The fungus, a kind of yeast, is found all over the world in soil. It can cause a kind of meningitis.

Aspergillus fumigatus is a kind of mold and has a mortality rate as high as 90%. You can find it almost anywhere that leaves fall on the ground and start to rot. Estimates suggest all of us inhale between 10 and 100 Aspergillus spores every day. 

The problem with Aspergillus and other fungal infections is they receive less attention and fewer resources than bacteria and viruses. Also, fungi are quick to adapt to medications and become resistant. Nevertheless, as many as 1.7 million people per year die from fungal infections, which is more than malaria and twice the number who die of breast cancer. There are over 150 million severe infections reported that are damaging but not fatal, as well.

Parasite Breakdown

Parasites are, hands down, the creepiest and most disturbing things that can infect you. Even if they aren’t as deadly as other infections, they tend to be more off-putting if for no other reason than many of them are big. These are living organisms that take up residence inside of you. The way they get inside of you can be just as disturbing as the fact they are inside you.

Take Strongyloides, for instance. This parasite transmits through feces but can live in soil for weeks. If you walk barefoot across it, they will burrow through the flesh of your foot. They’ll ride your bloodstream all the way to your lungs and then cause you to cough. Coughing brings them to your mouth where they get swallowed into your gut, right where they want to be. They can live for years in there and may turn deadly depending on the medications you take.

Giardia, a small parasite, is transmitted most often through feces or things contaminated with it. You can get it on your hands or in the food you eat and ingest it without realizing, as it’s a small, one-celled organism. 

Tapeworms, which can infect you if you accidentally ingest their eggs in undercooked meats, or from meat handled unhygienically, can grow to be up to 12 feet long in your gut, but some have been reported to be over 50 feet. They can live for 30 years.

Brain-eating amoebas have been contracted by people swimming in still, warm bodies of water like ponds in the US. They enter through the nose and infect the nervous system with a near 100% mortality rate. 

Parasitic infections can lead to sepsis among numerous other symptoms. One of the most well known and deadly parasites is the malaria parasite, transmitted by mosquitoes, which caused over 600,000 deaths in 2022. 

As disgusting as parasites can be, many of them will not actually kill you. It’s not in a parasite’s interest to kill its host, after all. It’s estimated about one in seven people in the world currently have an intestinal parasite. Some estimates boost that to about half the world’s population. 

So Which is Worst?

You can’t make a strong claim that any one of these deadly pathogens is better than another. There is far too much variety among each category to conclude one is preferable to another. In addition, there are so many mitigating factors that can alter just how dangerous an infection from one of these pathogens could be.

It’s much easier to state that you really don’t want to get an infection of any kind, be it parasitic, fungal, viral or bacterial. None of these are going to be a good time for you, and all should be avoided or treated quickly if they ever come up.

Being infected by parasites sounds like something that happens to other people in faraway places. However, parasites that infect humans are far more common than you might think. A lot of them have no symptoms, either, so people can go years without knowing they are infected.

Possibly up to 60 percent of the world’s population has at least one parasite in them right now. That’s right, there’s better than a 50/50 chance that you have one right now. Here is a list of ten of the most common asymptomatic parasites that could be nearer than you might think.

10 Tapeworms

Do you like your steak rare? Well, tapeworms (Taenia solium or Taenia saginata) get into human hosts from raw or undercooked meat. These tape measure–shaped parasites can grow up to 15 meters (50 ft) long. They like to live in people’s intestines and are transferred from animals, mostly cows and pigs. This happens either through the consumption of infected meat or unwashed vegetables.

If a tapeworm larva is eaten, it can grow into a full-blown adult that feeds off of the intestinal wall. This little guy can live in a human gut for nearly 30 years.^[1] People can grow old with their parasite. How cute. The tapeworm’s eggs, which can develop into cysts on organs, can be much more dangerous. If the infection is just the tapeworm (or worms), symptoms can be rare or do not appear at all. It will happily live unnoticed for years.

9 Liver Flukes

Liver flukes are parasites that infect the bile ducts and liver. They are one of a number of flatworms that breed in freshwater snails. One way they can infect humans is through the consumption of freshwater fish that share the same environment.

These flat-looking parasites can cause few symptoms in humans. Therefore, someone can be infected without ever knowing it. Mature flukes will eventually cause chronic inflammation of the bile ducts, which often leads to gallstones.^[2] While the majority of cases are found in developing countries, there have been cases in Hawaii, California, and Florida.

8 Hookworms

Two kinds of hookworms that can infect human hosts are Ancylostoma duodenale and Necator americanus. Hookworms make an interesting journey through the human body. They enter through the feet and can sometimes leave itchy rashes at their entry point. They will travel along the bloodstream until they can enter the lungs. Once in the lungs, they irritate lung tissue enough that the infected host coughs them up into their mouth. If even a little bit of that bloody, wormy phlegm gets swallowed, the hookworm is happily in its desired home: the small intestine.

This is a parasite that has been particularly prominent in some communities because it can be transferred from person to person through poop. If infected stool isn’t managed properly or is spread in soil, it can infect others easily. Therefore, now that we have better sanitation practices, hookworm infection has diminished, but it hasn’t disappeared.

It has been argued that the American South was left in an economic lethargy because of hookworms. For about 300 years, nearly 40 percent of the population was infected.^[3] If someone is heavily infected with hookworms, it can cause lethargy and mental incapacity. Therefore, with so much of the population infected, the South didn’t have the ability to keep up with its wormless neighbors. Otherwise, there are very few symptoms.

7 Pinworms

Pinworms are a parasite that can spread very easily. They are generally the most common parasitic infection in North America. They live on the human anus, so it takes just one scratch, and then anything that person touches can be covered with pinworm eggs. If someone swallows an egg, they become infected.^[4]

There are generally very few symptoms; sometimes it’s just a mild irritation, which encourages the infected host to scratch their butt and keep spreading the eggs. Mostly, pinworm infections are found in children, but they can easily infect anyone who has been exposed to them.

6 Ascariasis

Ascariasis is a roundworm infestation that attacks the small intestine. It usually spreads from exposure to human feces or uncooked meat, however it can also spread via human-to-human contact.^[5] Usually, it has few symptoms, unless the host has a large infestation of these roundworms.

Children are the most likely to display symptoms, and they’re often at a higher risk of getting infected with ascariasis because of their tendency to put their hands in their mouths. They can suffer bloated stomachs and pain as a result. Mostly though, the little roundworms just live in your stomach until they are ready to be pooped out.

5 Echinococcus Granulosus

Our canine besties can give us a parasite that will slowly grow inside humans for years without symptoms.

Echinococcus granulosus mainly infects dogs but can also get inside humans who have been exposed to canine feces or have even just petted a dog. It is a tapeworm-like larva that will create cyst-like lesions in the liver or lungs.^[6]

4 Trichinosis

Trichinosis is a roundworm infestation that usually occurs after eating undercooked or raw meat. It is mostly found in boar or pig meat. So if you’ve been hankering on having a medieval feast with your chums, make sure to cook that boar all the way through.

Once the trichinosis larva is in the intestines, it will develop into a mature worm, which will produce more larvae that can go through muscles and other body tissues.^[7] If a small sample of larvae is swallowed, the infected host might never know that they have trichinosis. However, a large infestation causes intestinal pain and diarrhea.

3 Dientamoeba Fragilis

This single-celled parasite is fairly mysterious. Not only do scientists and medical professionals not know how it is transmitted, but they don’t know if it produces any symptoms in human hosts.^[8] It might cause diarrhea and abdominal pain in some cases, but mostly, it is asymptomatic.

There have been some connections with pinworms, and it is conjectured that transmission comes from eating the eggs, but really we don’t know much about this pervasive parasite. We do know that Dientamoeba fragilis can infect large populations without anyone knowing they have a parasite in them.

2 Microsporidia

This group of single-celled parasitic fungi can infect a huge range of creatures, and humans are among them.^[9] Unless the human host is weakened in some manner (such as being immunocompromised), the fungi can exist asymptomatically and harmlessly. These spore-forming parasites often live in fish.

Microsporidia were originally considered to be protists. However, their genomes resembled fungi more than other eukaryotic organisms, and they are now classified as such.

1 Toxoplasma Gondii

This parasite might be one of the most famous, partly because it can be found in nearly a third of the world’s population and partly because it might alter someone’s brain, which sounds like something out of a science fiction novel. Toxoplasma gondii is a parasite aims to be in the gut of a cat. How it gets there is fascinating. It will infect rats and mice that have been exposed to cat feces. Then it will travel to the brain and pretty much take over the rodent’s natural fear of predators, turning it into attraction. The rodents, instead of running away like they normally would do, instead seek out cats and promptly get eaten. The parasite has then succeeded in its goal of ending up in a cat’s gut.

Where it gets weird is that Toxoplasma gondii can also infect humans. There are generally no symptoms unless the person infected is pregnant. However, the parasites might also try to change the brain chemistry of their human hosts, potentially leading to schizophrenia and suicide.^[10] At least one scientist attributes Toxoplasma gondii with political dissent and higher rates of car crashes. This, however, hasn’t been accepted by mainstream scientists. But maybe next time you forget to pay your bills or walk into traffic, you can blame your cat’s parasites.

Hannah Storrs is a freelance writer who has written primarily for hiking websites like The Trek. She is based out of Montreal, QC. If you like her work, you can find more at https://www.hannahstorrs.com/

Parasites and pathogens are pretty terrifying things. Immune systems serve to mount a defense against these elusive little creatures. This is, in fact, how sexes came to be, as a means to mix the genes necessary to stir up the genetic pot, creating more stout and powerful immune systems as various microscopic invaders adapted to override the host’s bodily defenses.^[1]

This evolutionary arms race has been a battle between predators and prey of all sizes and on all scales to come up with the most effective way to survive and procreate. Whenever one side gets stronger, the other adapts and comes up with a new, clever way to beguile its foe and succeeds for a while—until it’s inevitably outdone again. This has also happened with many plagues, like the bubonic plague, throughout the long path of history.

These days, parasites and pathogens have developed some pretty interesting ways to procreate and/or move on to the next stage of their life cycle—including hijacking the minds of their hosts. These bodily invaders control their hosts’ behavior, forcing them subtly into actions which will result in an opportunity for them to spread or reproduce. Here are ten parasites and pathogens which control the minds of humans.

10 Trypanosoma Brucei

Trypanosoma brucei is a species of protozoa. It is a blood parasite that infects a slew of animals and occasionally humans, too. Its life cycle is rather long, starting off in tsetse flies, which bite humans. Then it enters into the human’s lymphatic system, and from there, it transfers into the bloodstream.

An infection from this parasite can cause sleeping sickness, which can harm both animals and people and comes in two separate stages of symptoms. The early onset of the infection comes on like many other diseases, with joint pain, muscle pain, fever, and swollen lymph nodes, while the second stage causes behavioral changes and extreme lethargy as the parasite begins to attack the spine and brain. Ultimately, T. brucei can kill you.

It should be noted here that the goal of many of these bloodstream parasites seems to be to render its host compromised without killing it. A dead host isn’t as likely to spread the parasite and help complete the life cycle, so rather than killing indiscriminately, it’s advantageous for a parasite to simply weaken its host, making it the potential prey of other animals which are necessary for the parasite to reproduce.^[2]

9 Intestinal Bacteria

Yes, the very same intestinal bacteria that you’ve likely had your entire life and have almost never even thought about is capable of causing some pretty unusual changes in your mental state. Furthermore, these bacteria can play a pretty vital roll in very human problems, such as depression and anxiety, it seems.^[3] Science has long noted the link between microbiota, the bacteria living in the gut, and animal behavior, mainly in rodents and chimpanzees.

But recent human studies have divided people into distinct groups based on the presence of different bacteria in their guts in starkly different amounts to determine the potential impact of intestinal bacteria on human mood. They monitored the subjects with fMRI machines as well as other equipment to record the responses of their brains to imagery. One group had more Bacteroides, while the other group had more Prevotella, two genera of bacteria that live in the intestines of humans and are thought to alter mood.

When showed images of emotionally charged material, the brains of those in the Prevotella group lit up, indicating that they were responding more intensely. Beyond this, the Prevotella group presented more anxiety and depression, as well as other negative emotions. While this work is far from definitive, it’s extremely safe to assume that, like our primate relatives, gut bacteria plays a roll in the regulation of the moods of humans as well.

8 Toxoplasma Gondii

Toxoplasma gondii, the parasite which causes the disease known as toxoplasmosis, passes through both humans and cats. Beyond being just troublesome, especially in small children, it can also control the minds of the organisms it inhabits. Toxoplasmosis can do some serious damage to those with compromised immune systems, such as the elderly, HIV patients, and those with other diseases which cause one to become immunocompromised. Even though the parasite is only believed to reproduce when it’s inside cats, it still manages to make its way into humans through the feces of cats (when handled) as well as when it infects other animals that we consume.

This parasite affects the behavior of rats, mice, and other rodents. Seeing as they only reproduce inside the bellies of cats, they hijack the minds of the rodents, which are commonly prey for cats, and give these animals a sense of fearlessness, making them unafraid of their cat predators.

But research shows that this parasite also affects the behavior of humans as well. Studies suggest that it promotes risky behaviors in people, much like the rodents, and causes other marked behavioral changes. Not only are people infected with the parasite willing to take on more risky life ventures, but experiments have shown that they are even more willing to drink a surprise, mystery fluid when presented it by scientists, without being told what it is, and are generally willing to commit themselves to other unusual, risky behaviors. It seems that the natural human skepticism is reduced in those whom Toxoplasma gondii calls home.^[4]

7 More Intestinal Microflora

Yet again, we find that microorganisms in the gut can control the minds of their host humans. This time, it’s not the mood that these life-forms can affect or modulate but cravings. For instance, some people love chocolate, while others are more indifferent. The latter can actually have gut bacteria which are sort of “immune” to chocolate—that is, the microbes don’t like it very much, and thus, they don’t cause the cravings. These bacteria can have some pretty far-reaching effects: Studies have shown that, all things being equal, obese people have gut bacteria that are different and distinct from people of a more moderate weight.

Sugar cravings, in a way, actually feed themselves by feeding the organisms that cause them. Candida is a type of yeast that grows in the gut and particularly loves to feed on the sugars that we take in.^[5] When these little fungi grow too much, they emit chemicals which are likely to cause the person to crave more sugar, thus continuing the cycle of the microflora themselves. In a weird way, they hijack your mind to give you sugar cravings because they’re having sugar cravings, and they have evolved to emit chemicals which cause you to have the same sugar cravings that they do—so you’ll feed them.

6 Strep Throat

Strep throat, or rather the bacteria that cause it, can lead to some pretty unusual and sometimes lasting behavioral changes in people, especially children. Over the years, science has begun to tie together the link between strep throat and ongoing behaviors that sometimes, in some rare cases, seem to last. In most people, antibiotics or the immune system simply clear strep throat, and they go on with their lives, but that isn’t always the case. Sometimes, children suffer from nervous tics and even full-blown obsessive-compulsive disorder after the pathogen takes hold of them.

This condition is known as PANDAS, short for pediatric autoimmune neuropsychiatric disorder associated with streptococcal infection. It can present as some pretty severe anxiety and other mood disturbances, like separation anxiety or a massive fear of bugs or germs. While OCD and other disorders tend to develop over time, PANDAS comes on seemingly overnight and strikes without warning. This leads researchers and doctors to believe it has something to do with strep throat controlling and affecting the mind of its host.^[6]

5 Rabies

When people think of a disease that alters the minds of animals and people, usually rabies comes to mind, if not Toxoplasma gondii. Rabies is a virus that affects the brain and spine, thus dealing some pretty hefty damage to its host organism before almost always killing it.^[7] Rabies lives in the saliva of infected humans and animals, which then transmits to other hosts when one bites the other.

Humans who are bitten go through some interesting behavioral changes which help the virus reproduce, just like what happens with other animals. Organisms carrying the rabies virus tend to become hyperaggressive and agitated, and most mammals become more brazen and willing to bite, even becoming unusually brave to do so. Humans can suffer delirium and hallucinations as well as flu-like symptoms at first. But when the disease takes hold, the virus is almost always fatal, with fewer than ten people reported surviving the clinical stage of rabies in the United States—ever.

Even more bizarre is that the rabies virus causes hydrophobia, an extreme fear of water. Seeing as rabies lives in the saliva of the infected, this makes complete sense—hosts which are afraid of water won’t wash the virus out of their mouths, making it more capable of transmission and reproduction. The evolutionary arms race at work.

4 Naegleria Fowleri

Naegleria fowleri is a terrifying little critter, an amoeba that goes straight for the brain upon its infection of its host. It is also known as the brain-eating amoeba, and it feasts on bacteria. Even scarier, it lives in water and can travel up through the nose and into the brain, where it does its damage, usually ultimately killing a person. A simple trip to the lake or even contact with water from around your dwelling can expose you to this parasite.

The initial symptoms of this tiny beast begin anywhere between one and nine days after exposure, usually starting at about five days, and can include headache, nausea, vomiting, and basic flu-like symptoms at first. But then in can develop into a lack of attention to people and surroundings, as well as vertigo or loss of balance, hallucinations, and eventually death.^[8]

3 Malaria

One of the most brutal and unforgiving diseases of all time, malaria also has one of the most interesting life cycles. Malaria is transmitted largely through mosquito bites. When a female mosquito bites a person carrying malaria, they get it, and then they go bite someone else and spread it to them. Pretty simple. See, Plasmodium falciparum and Plasmodium vivax, two of the five species of Plasmodium which cause malaria in humans, spend part of their life cycle in humans and the other part in mosquitoes, making it essential for the organism to transmit to both mosquitoes and humans.

But where this process gets interesting is the cravings that malaria can cause in its host organisms. Malaria also relies heavily on sugar, the main substance of the mosquito diet, to complete its life cycle, and mosquitoes actually bite you to obtain your blood’s sugar. The parasites themselves are also evidenced to survive on sugar in each organism, both the mosquito and the human being. Aside from human blood, mosquitoes live largely on nectar and other plant sugars found in the wild to survive.^[9]

Malaria has been demonstrated to not only make mosquitoes more hungry, giving them a case of the mosquito munchies, but also give the mosquitoes cravings during various periods of the incubation of the malaria parasites. During the period where the parasite needs to be inside the mosquito, the mosquito will crave the sweet scent of plant nectar, and thus the parasite remains. When it comes time to be transmitted into a human, the mosquito begins to crave human blood and then feasts on a person to aide in the life cycle of the zombie parasite. But that’s not all. Malaria in humans eats up the sugar and hormones in the blood pretty quickly, which can lead to low blood sugar, but it also leads to anemia and vitamin deficiency. And guess what those vitamin deficiencies and anemia cause in humans—sugar cravings.

Malaria controls the mosquito when in the mosquito, giving it cravings for its plant food while the parasite incubates and then making it crave blood when it’s time to travel into a human. From there, malaria eats the sugar in the blood but also causes anemia and vitamin deficiencies, which will cause sugar cravings, which will lead to the human boosting blood sugar levels so that the malaria can get back into the mosquitoes again.

2 Chlorovirus ATCV-1

This nasty little virus has long been known to affect the behavior patterns of mice, causing some pretty severe cognitive deficiencies in them, and it is also known to infect humans. There is a lengthy process by which this virus makes a slew of chemical changes which affect the behavior of its host organisms, but in short, it makes people dumb. Yes, it’s a stupid virus.^[10]

Chlorovirus ATCV-1 significantly impairs the cognitive abilities of humans who are infected with it, and if that wasn’t scary enough of an idea, this virus can live inside you for years. Beyond that, there was a small study on the virus in the United States, which concluded that 44 percent of the participants, in fact, had the virus, which typically lives in algae but tends to reside in the throat in humans. So there actually is a stupid virus. Who would have thought?

1 Influenza

Science is learning new things every day about human behavior, and in today’s world of vaccines, how humans respond to them is no exception. It’s coming to light that flu vaccines actually increase the likelihood of humans becoming social—that is, the flu vaccine makes humans more interested in socializing. Even more interesting than this, studies have noted that the flu spreads through our social networks (the real-life ones, not the online ones) and varies depending on which network you’re exposed to. Who you hang out with might be a determining factor in whether or not you’ll catch one of the most devastating diseases in world history.

But beyond just the vaccine, the influenza virus also hijacks the mind of its host subtly, which is likely why people who’ve just received the vaccine demonstrate the same phenomenon. The flu itself makes people want to be more social, which makes perfect sense, as a socially outgoing host is a perfect way for the virus to spread to other people.^[11] While the mechanisms aren’t quite clear yet, we do know that people with the flu become more interested in seeking out and engaging with other people, and considering how often parasites and pathogens control the minds of their hosts, its quite probable that we’ll learn the mechanism through which the flu makes us want to fraternize so that it can come out to play.