The human brain has earned the reputation of being the most complex structure in the universe. It’s a bold claim, but science backs it up. Nothing else we know matches its intricacy, and that makes the question “how many behavior‑altering organisms exist?” all the more chilling. Our grey matter is delicate, and countless microscopic hitchhikers can slip in, mess with our neural circuits, and change the way we act.

how many behavior are we really facing?

1 Wolbachia the Gender Manipulator

Wolbachia bacteria specialize in taking over insect hosts, and they do it in a way most of us would find astonishing. Once inside an insect, these microbes can rewrite the host’s reproductive script, skewing the sex ratio of the offspring. In the Ostrinia scapulalis moth, for instance, Wolbachia hijacks the chromosomes so that male embryos are eliminated, leaving only females to hatch.

Transmission of Wolbachia is strictly maternal – the bacteria ride inside the egg that a female lays. They never travel through sperm, so the parasite’s survival hinges on having plenty of females around. By wiping out as many males as possible, Wolbachia boosts the odds that its host will be a female, which in turn guarantees another generation of infected eggs. It’s a ruthless but effective reproductive hack.

Fewer males also means reduced competition for food and resources, giving Wolbachia‑carrying females a better shot at thriving. This manipulation is so successful that over half of all arthropod species are now known to harbor Wolbachia, turning a huge swath of the insect world into a laboratory for bacterial gender control.

2 Cordyceps Makes Zombies

If you’ve ever watched The Last of Us, you already know the terrifying image of a fungus turning people into grotesque, mind‑less husks. In reality, cordyceps fungi pull off a similar stunt, just on far simpler creatures. Each cordyceps species has a single preferred host – some target ants, others go after spiders, moths, or even beetles – and they never stray beyond that one niche.

When an ant becomes infected, the fungus stealthily spreads through its body, growing until it’s large enough to pierce the exoskeleton. It then sprouts delicate filaments across the ant’s surface, deliberately avoiding vital organs so the host stays alive long enough to serve the parasite’s needs.

These filaments act like microscopic tendrils that weave through the ant’s muscles, effectively hijacking its locomotion. The fungus essentially puppeteers the insect, forcing it to move in ways it never would have on its own.

The real mind‑bender happens when the fungus infiltrates the ant’s brain. It rewires neural pathways, compelling the insect to abandon the safety of its nest and climb to a spot ideal for spore dispersal – often a leaf or twig at just the right height. There, the ant clamps its mandibles onto the substrate and awaits death.

By the time the ant reaches this death perch, the fungus has commandeered more than half of its brain tissue. It alters serotonin levels, messes with dopamine, and disrupts the chemical cues ants normally use to communicate. Even the ant’s sense of time is scrambled, making it leave the nest at odd hours.

Neurotransmitters that trigger hallucinations, spasms, and hyper‑activity also surge, turning the ant into a zombie‑like climber. The fungus even generates enzymes that destroy the ant’s jaw muscles, ensuring the insect can’t open its mouth and escape. Meanwhile, the host’s immune system is suppressed, allowing the fungus to replace nearly every cell with its own tissue.

Other insects suffer a similar fate. Spider‑infecting cordyceps, for example, forces its arachnid host to ascend a plant stem, while moth‑targeting strains make the moth crawl upward. In each case, the parasite’s ultimate goal is a high perch where spores can rain down on unsuspecting victims below.

Fortunately for us, the fungus can’t survive at human body temperature, and our complex brains are far beyond its reach. So while cordyceps are master puppeteers in the insect world, they’re not a threat to human civilization – at least not yet.

3 Lancet Liver Flukes Also Zombify Ants

If you thought one zombie‑making fungus was enough, meet the lancet liver fluke. Ants that nibble on the tiny fluke larvae end up with a single worm making its way straight to their brain, while its siblings hide in the ant’s stomach. Once the brain‑resident fluke takes hold, it reprograms the ant’s behavior.

The infected ant is compelled to climb a blade of grass, latch onto it with its mandibles, and stay put – essentially turning the ant into a living fishing hook for the next animal in the chain. When a grazing herbivore, such as a cow, chews the grass, it inadvertently swallows the immobilized ant, delivering the flukes straight to its own liver.

Inside the herbivore’s liver, the flukes mature and lay eggs, which are later expelled in the host’s feces. Those droppings become a banquet for snails, which ingest the eggs. Inside the snail, the flukes hatch, multiply, and are eventually released in a mucous‑laden “ball” that attracts searching ants, completing the macabre life cycle.

4 Horsehair Worms Cause Suicide

You’ve probably seen viral clips of a horsehair worm bursting out of a praying mantis or cricket, looking like a strand of actual horse‑hair. These parasites are grotesquely long – some stretching over a foot – and they need water to complete their development.

When the worm reaches adulthood inside its insect host, it hijacks the host’s nervous system, forcing the creature to plunge into a nearby body of water. Once submerged, the parasite tears itself free through the host’s posterior, emerging in a dramatic, often gruesome display.

Sometimes hundreds of these foot‑long worms emerge in a tangled knot, while the host’s body may still be twitching. The freed worms then release their eggs into the water, where they are eaten by insect larvae, restarting the cycle of infection.

5 Toxoplasmosis Removes Fear

About one‑third of the global population – billions of people – carry the protozoan parasite Toxoplasma gondii, which is most famously linked to cats. In rodents, the parasite takes a terrifyingly direct approach to ensuring its own reproduction.

When a mouse or rat becomes infected, the parasite migrates to the brain and dampens the animal’s innate fear of felines. It even heightens attraction to cat urine, making the rodent more likely to wander into a cat’s path, where it gets devoured, allowing the parasite to complete its sexual cycle inside the feline’s gut.

Humans can pick up the infection by handling cat litter, consuming contaminated soil, or eating undercooked meat. Once inside a person, the parasite’s eggs hatch in the stomach, the larvae cross the intestinal wall, and travel via the bloodstream to settle in the eyes and brain. In healthy individuals, the parasite usually remains dormant and causes few symptoms.

However, severe cases have been associated with psychiatric conditions such as schizophrenia, heightened aggression, and even suicide, though a direct causal link remains unproven. Pregnant people are especially cautioned, as the parasite can cross the placenta and harm the developing fetus, which is why changing cat litter is discouraged during pregnancy.

Recent studies suggest that even in otherwise healthy hosts, Toxoplasma may subtly influence behavior, nudging infected individuals toward riskier decisions. The parasite’s ability to tweak the human brain, even faintly, adds another layer to the ever‑growing list of behavior‑altering microbes.

6 Myrmeconema Neotropicum Parasites Change Their Host’s Appearance

Deep within Central American rainforests, a roundworm known as Myrmeconema neotropicum has turned ordinary black ants into eye‑catching red‑bellied look‑alikes of the local berries. Researchers observed infected ants sporting enlarged, crimson abdomens that made them stand out against the forest floor.

The theory is that these vivid ants mimic ripe berries, luring frugivorous birds to pluck them. When a bird snaps up the ant, the parasite’s eggs travel through the bird’s digestive system and are deposited in its droppings, which later fall back onto the forest floor, ready to infect more ants.

Scientists are still piecing together exactly how the worm achieves this dramatic color shift. It may interfere with the ant’s melanin production, introduce a novel pigment, or both. Additionally, infected ants exhibit a markedly thinner cuticle – up to 25 % thinner – as the parasite feeds on their exoskeleton while simultaneously re‑shaping their appearance.

10 ways 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.

10 The 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 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.

9 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 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.

8 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 the 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.

7 Virotherapy

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.

6 Using 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 antibiotics, however, 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.

5 Vaccines

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.

4 Bacterial 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.

3 We 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.

2 Skin 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 Staphylococcus epidermidis, 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 Staphylococcus 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.

1 Life 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.

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, while others feel like a death sentence the instant they’re diagnosed.

What 8217 s the Most Dangerous Microbe?

1 The Basics

Generally speaking, a virus tends to be more hazardous than a bacterium, though that’s a blanket statement that comes with a big “but.” The common cold virus is far less threatening than, say, botulism‑producing bacteria. Context matters.

Bacteria are single‑celled organisms that can survive on their own. Most are harmless, and many actually help us—our gut alone hosts roughly 100 trillion bacteria that aid digestion. Only a tiny fraction cause trouble. In size, bacteria are roughly ten to a hundred times larger than viruses, ranging from one to three microns, with Salmonella as a familiar example.

Viruses, on the other hand, can’t live independently. They act like parasites, hijacking a host’s cells to reproduce, which often results in illness or death. Their size is minuscule—about 20 to 200 nanometers across.

Parasites belong to the eukaryote kingdom, meaning they have a nucleus and internal structures, making them larger than both viruses and many bacteria. Some parasites are entire organisms, like tapeworms, that take up residence inside us.

Fungi most often appear as spores or molds. A common example is athlete’s foot, a fungal infection that thrives in damp environments.

2 Bacteria Breakdown

A solitary bacterium is a fully formed, single‑cell microbe capable of surviving outside the human body. They flourish in soil, rotting food, on skin—anywhere conditions are right.

The most dangerous bacteria can wreak havoc in several ways. Many produce deadly toxins that can paralyze or outright destroy our cells, disrupting normal function. Others multiply so aggressively that they outcompete healthy cells for resources.

Antibiotics have revolutionized medicine by either killing bacteria or halting their growth. They achieve this by either breaking down the bacterial cell wall or interfering with the organism’s ability to reproduce.

Because bacteria reproduce rapidly—every 20 to 30 minutes in ideal conditions—they also mutate quickly. This rapid evolution has given rise to antibiotic‑resistant strains. Some bacteria produce enzymes that deactivate antibiotics; others pump the drugs out before they can act.

Common culprits like Salmonella, gonorrhea, and Campylobacter have developed resistant strains, turning once‑easily‑treated infections into potentially lethal threats.

The ever‑changing nature of bacteria makes naming a single “worst” organism impossible. In 2024, the World Health Organization highlighted 15 drug‑resistant bacteria as especially dangerous. Near the top sits Mycobacterium tuberculosis, the bacterium behind TB, responsible for roughly 1.7 million deaths each year.

3 Virus Breakdown

Viruses aren’t cells or independent living entities. They consist of a tiny packet of genetic material wrapped in protein. Outside a host, a virus is inert—without a living cell to commandeer, it can’t replicate or cause disease.

Once a virus infiltrates a host, it hijacks the host’s cellular machinery to make copies of itself. This process often destroys the host’s cells, leading to infection. Their minute size even allows them to infect bacteria and fungi, and they can be inhaled, transmitted via insects, or spread through bodily fluids—pathways unavailable to larger microbes.

The immune system attempts to generate antibodies to neutralize the invader. If the virus replicates faster than the body can mount a defense, illness ensues and can be fatal. The viral replication cycle inherently damages host cells.

A fever is one of the body’s first defenses; many viruses can’t survive the elevated temperature, though prolonged fevers can be dangerous for the patient as well.

Antibodies are the second line of defense, but they require prior exposure to a pathogen to be produced. When encountering a novel virus, the immune system may be caught off‑guard.

Ebola is a stark example of extreme lethality, with mortality rates reaching up to 90 percent. Its rapid, deadly course, however, limits its spread compared with less‑virulent viruses.

HIV, by contrast, has spread worldwide and has claimed around 32 million lives. Modern antiretroviral therapies have dramatically reduced mortality, but the virus remains a historic heavyweight.

The 1918 influenza pandemic, often called the Spanish Flu, caused an estimated 50‑100 million deaths globally, underscoring how a seemingly ordinary virus can become catastrophic.

Rabies is another terrifying virus—once symptoms appear, the fatality rate is essentially 100 percent without prompt treatment.

Viruses that humanity has largely eradicated, such as smallpox, once killed roughly 300 million people before vaccination campaigns eliminated them.

4 Fungi Breakdown

Pop‑culture has turned fungal infections into something of a horror‑movie staple. Articles about bizarre fungi eventually inspired the video‑game series The Last of Us, where a cordyceps‑type fungus decimates humanity.

In reality, cordyceps infect insects, forcing them to climb and cling to vegetation before the fungus erupts from their bodies. Humans, with far more complex immune systems, are not susceptible to this particular pathogen—unless it somehow mutates.

Other fungi, however, pose real threats. In 2023, the CDC warned about Candida auris, a drug‑resistant yeast that spreads in hospitals and can invade the heart, lungs, bloodstream, eyes, bones, and other organs.

Cryptococcus neoformans, another ubiquitous yeast found in soil, can cause meningitis with mortality rates between 41 % and 61 %, especially in immunocompromised patients.

Aspergillus fumigatus, a common mold that thrives on decaying foliage, carries a mortality rate as high as 90 % in invasive infections. Everyone inhales dozens of spores daily, but most remain harmless—unless the immune system is weakened.

Fungal infections receive far less research funding than bacterial or viral diseases, yet they claim roughly 1.7 million lives each year—more than malaria and double the deaths from breast cancer. Over 150 million severe, non‑fatal fungal infections are reported worldwide.

5 Parasite Breakdown

Parasites are arguably the most unsettling microbes. While not always fatal, their size and life cycles make them especially creepy. They are living organisms that settle inside a host, often entering through disturbing routes.

Take Strongyloides, a nematode that thrives in contaminated soil. Walking barefoot can let its larvae burrow through the skin, travel via the bloodstream to the lungs, trigger a cough, and then be swallowed back into the gut, where they can reside for years, potentially turning lethal if the host’s immunity falters.

Giardia, a microscopic parasite, spreads through fecal‑contaminated water or food. Ingesting cysts leads to diarrheal illness, especially in areas with poor sanitation.

Tapeworms, contracted by eating undercooked meat harboring eggs, can stretch up to 12 feet inside the intestine, with some rare cases exceeding 50 feet and persisting for decades.

Brain‑eating amoebas, such as Naegleria fowleri, infiltrate the body through the nose when swimming in warm, stagnant water, leading to a near‑100 % fatality rate.

Parasites can also trigger sepsis and a host of other complications. Malaria, caused by Plasmodium parasites transmitted by mosquitoes, resulted in about 600 000 deaths in 2022 alone.

Most parasites don’t aim to kill their host; they need a living environment to survive. Roughly one in seven people worldwide harbors an intestinal parasite, and some estimates suggest up to half of humanity carries one at any given time.

6 So Which Is Worst?

It’s impossible to crown a single pathogen as the absolute worst. Each category—bacteria, parasites, fungi, viruses—contains a dizzying array of organisms with wildly different traits, transmission methods, and mortality rates. Variables such as geography, health status, and access to medical care dramatically shift the danger level.

The safest advice is simple: avoid infection whenever possible, regardless of the microbe. Prompt diagnosis and treatment are essential if you ever find yourself infected, whether the culprit is a bacterium, virus, fungus, or parasite.

Thinking parasites only belong in far‑off jungles is a common myth. In reality, human‑infecting parasites are surprisingly prevalent, often slipping past us without a single hint. Many of these tiny hitchhikers cause no obvious signs, letting people coexist with them for years without ever realizing it.

The Top 10 Parasites You May Be Carrying

10 Tapeworms

Ever wonder what a 50‑foot ribbon in your gut would feel like? Tapeworms—specifically Taenia solium and Taenia saginata—sneak into humans when we bite into raw or undercooked meat. These flat, tape‑measure‑shaped parasites can stretch up to 15 metres (about 50 ft) and make a cozy home in our intestines after we eat infected pork or beef, or even unwashed veggies tainted with eggs.

Once a larva is swallowed, it unfurls into a full‑grown adult that attaches to the intestinal wall, siphoning nutrients for up to three decades. While the worm itself may be silent, its eggs can migrate and form cysts in organs, posing a greater danger. Many carriers never notice a thing, allowing the tapeworm to live undisturbed for years.

9 Liver Flukes

Liver flukes are flatworms that take up residence in the bile ducts and liver. Their life cycle begins in freshwater snails, and they can jump to humans when we eat fish from the same waters that harbor the infected snails.

These parasites often cause barely any symptoms, letting an infection fly under the radar. Over time, mature flukes can inflame the bile ducts, leading to gallstone formation. While they’re most common in developing regions, cases have popped up in places like Hawaii, California, and Florida.

8 Hookworms

The two main human hookworms—Ancylostoma duodenale and Necator americanus—embark on a bizarre journey. They breach the skin, often through bare feet, causing a fleeting itchy rash, then travel via the bloodstream to the lungs. There, they irritate lung tissue, prompting a cough that brings them up the throat, where swallowing deposits them in the small intestine.

Historically, hookworms spread easily through contaminated soil and feces, especially where sanitation lagged. In the American South, an estimated 40 % of the population grappled with hookworm infection for three centuries, contributing to widespread lethargy and reduced productivity. Modern sanitation has curbed the prevalence, but the parasite still lingers in some regions.

Heavy infections can sap energy and impair cognition, yet many carriers experience only mild or no symptoms, making the worm a stealthy adversary.

7 Pinworms

Pinworms rank as the most common intestinal parasite in North America. They reside around the anus, laying countless microscopic eggs that cling to fingers after a scratch. A single touch can spread these eggs to surfaces, toys, or bedding, and ingestion of an egg leads to a new infection.

Symptoms are usually mild—perhaps a subtle itch that prompts nightly scratching—so many people never suspect they’re infected. While children are the typical victims, anyone exposed to contaminated hands or objects can become a host.

6 Ascariasis

Ascariasis is caused by the roundworm Ascaris lumbricoides, which settles in the small intestine. Transmission usually occurs through contact with contaminated soil, feces, or undercooked food, and the parasite can also spread directly from person to person.

Most infections are silent, but a heavy worm load can balloon the abdomen, cause pain, and interfere with nutrient absorption. Children are especially vulnerable because they’re prone to putting dirty hands in their mouths, turning a playground into a worm‑laden arena.

5 Echinococcus Granulosus

Our canine companions can inadvertently pass a stealthy parasite to us. Echinococcus granulosus primarily infects dogs, but humans can pick it up by handling dog feces or simply petting a contaminated pup.

The parasite’s larval stage forms slow‑growing cysts in the liver or lungs. These cysts may linger for years without causing noticeable problems, making the infection a quiet long‑term threat.

4 Trichinosis

Trichinosis stems from eating undercooked pork or wild boar that harbors the larvae of Trichinella spiralis. When these larvae reach the intestines, they mature into adult worms that release new larvae, which then migrate into muscle tissue.

A tiny dose of larvae may slip by unnoticed, but a larger infestation triggers intestinal pain, diarrhea, and muscle soreness as the parasites invade the body’s fibers. Cooking meat thoroughly eliminates the risk.

3 Dientamoeba Fragilis

Dientamoeba fragilis is a single‑celled organism that remains something of a mystery. Scientists still debate how it spreads, and whether it truly causes disease. Some infected people report diarrhea and abdominal cramps, but many carry the parasite without any signs.

There’s a hinted link to pinworm infections, and some researchers suspect that eating pinworm eggs might be a transmission route. Regardless, large populations can harbor D. fragilis silently.

2 Microsporidia

Microsporidia are a group of spore‑forming, single‑celled fungi that can infect a wide array of hosts, humans included. In healthy individuals they often sit quietly, causing no symptoms, but in immunocompromised patients they can provoke serious illness.

These organisms were once classified as protists, but genetic analysis revealed a closer kinship to fungi. They’re commonly found in fish and can be transmitted through contaminated water or food.

1 Toxoplasma Gondii

Toxoplasma gondii is arguably the most infamous of the lot, infecting roughly one‑third of the global population. Its life cycle centers on cats, but the parasite can hop onto rodents via cat feces, then manipulate the rodent’s brain to make it less fearful of felines—essentially turning the mouse into a cat‑magnet.

Humans can acquire the parasite by ingesting oocysts from contaminated soil, water, or undercooked meat. Most infections are asymptomatic, but pregnant women risk severe fetal complications. Some studies even suggest subtle behavioral changes in infected adults, linking the parasite to increased risk of schizophrenia, suicide, or even altered driving habits—though these claims remain controversial.

Author Note: Hannah Storrs is a freelance writer who has contributed to outdoor and travel publications. Find more of her work at https://www.hannahstorrs.com/.

Parasites and pathogens are downright spooky. Our immune systems constantly battle these sneaky invaders, which have even driven the evolution of sexual reproduction as a way to shuffle genes and build tougher defenses against ever‑changing microscopic foes.^[1] This relentless arms race has produced a dazzling array of strategies, including the ability to pull the strings of their human hosts’ minds. Below are ten parasites and pathogens that have mastered the art of mind control.

10 Trypanosoma Brucei

Trypanosoma brucei is a protozoan blood parasite that infects a wide range of animals and, on occasion, humans. Its life cycle begins when a tsetse fly bites a person, injecting the parasite into the lymphatic system, which then migrates into the bloodstream.

The infection triggers sleeping sickness, a disease that unfolds in two distinct phases. Early symptoms resemble many other illnesses—joint and muscle aches, fever, and swollen lymph nodes—while the second stage brings dramatic behavioral shifts and profound lethargy as the parasite attacks the spinal cord and brain. Ultimately, T. brucei can be fatal.

Crucially, many bloodstream parasites aim to debilitate rather than immediately kill their hosts. A dead host can’t spread the parasite, so weakening the host makes it more vulnerable to predation by other animals essential for the parasite’s reproduction.

9 Intestinal Bacteria

Yes, the very same gut bacteria that have lived inside you for years can subtly reshape your mental landscape. These microbes are linked to mood disorders such as depression and anxiety. Decades of research have shown a relationship between microbiota and behavior in rodents and chimpanzees.

Recent human studies divided participants into groups based on the prevalence of two bacterial genera: Bacteroides and Prevotella. Using fMRI, researchers observed that the Prevotella group exhibited heightened brain activity when shown emotionally charged images, indicating a stronger emotional response.

Moreover, the Prevotella cohort reported higher levels of anxiety and depressive symptoms. While the evidence is not yet conclusive, it strongly suggests that, like our primate relatives, gut microbes play a role in regulating human mood.

8 Toxoplasma Gondii

Toxoplasma gondii causes toxoplasmosis and shuttles between cats and a variety of warm‑blooded hosts, including humans. While the parasite reproduces exclusively in cats, it can infect humans through cat feces or by consuming undercooked meat.

In rodents, the parasite erases the innate fear of cat scent, rendering them bold and more likely to be preyed upon—a perfect strategy for reaching its feline definitive host.

Human studies suggest similar effects: infected individuals display a propensity for riskier behavior, a reduced aversion to novel, potentially dangerous situations, and an increased willingness to consume mystery fluids in laboratory settings. In short, T. gondii appears to dampen the usual human skepticism.

7 More Intestinal Microflora

Beyond mood, gut microbes can hijack our cravings. Some people adore chocolate, while others are indifferent; this difference can stem from distinct bacterial populations. Certain microbes are “immune” to chocolate, meaning they don’t trigger cravings.

Research shows that obese individuals often harbor a different gut microbiome composition compared to those of average weight. One particular yeast, Candida, thrives on dietary sugars. When it overgrows, it releases chemicals that stimulate the host to crave more sugar, creating a feedback loop that feeds the fungus.

In essence, these microorganisms manipulate the brain’s reward pathways to ensure they receive the nutrients they crave, turning the host into an unwitting accomplice.

6 Strep Throat

Streptococcal bacteria, the culprits behind strep throat, can sometimes trigger lasting neuropsychiatric changes, especially in children. While most infections resolve with antibiotics, a subset of youngsters develop sudden‑onset tics, obsessive‑compulsive behaviors, and intense anxiety.

This phenomenon is known as PANDAS—Pediatric Autoimmune Neuropsychiatric Disorder Associated with Streptococcal infections. Symptoms may include severe separation anxiety, an overwhelming fear of germs, and other obsessive‑compulsive traits that appear almost overnight.

The rapid emergence of these symptoms points to an immune‑mediated attack on the brain, suggesting that the bacteria indirectly “control” the host’s mind.

5 Rabies

Rabies is the classic mind‑altering virus that attacks the central nervous system. The virus resides in the saliva of infected animals and spreads through bites.

Infected hosts become hyper‑aggressive, unusually brave, and prone to biting—behaviors that dramatically increase transmission opportunities. Humans may also experience delirium, hallucinations, and flu‑like symptoms early on. Once clinical rabies sets in, the disease is almost invariably fatal; fewer than ten U.S. cases have survived the clinical stage.

One of the most bizarre symptoms is hydrophobia—a terrifying fear of water. This aversion prevents the host from washing away the virus from its mouth, thereby enhancing the pathogen’s chances of spreading.

4 Naegleria Fowleri

Naegleria fowleri, often dubbed the “brain‑eating amoeba,” lurks in warm freshwater and can invade the brain via the nasal passages. After a brief incubation of 1‑9 days, initial symptoms mimic a flu: headache, nausea, and vomiting.

As the infection progresses, victims may lose awareness of their surroundings, experience vertigo, suffer balance issues, and endure vivid hallucinations before the disease inevitably proves fatal.

Because the organism thrives in warm, stagnant water, simple activities like swimming in lakes or using contaminated tap water can expose unsuspecting individuals to this deadly pathogen.

3 Malaria

Malaria, caused by Plasmodium species, is transmitted by the bite of infected female mosquitoes. The parasite’s life cycle straddles both humans and mosquitoes, demanding precise timing to ensure successful propagation.

Research shows the parasite tweaks the host’s cravings. In mosquitoes, the parasite induces a “mosquito munchies” effect, heightening the insect’s desire for plant nectar during the parasite’s developmental phase. When it’s time for transmission, the same parasite drives the mosquito to crave human blood, prompting a bite.

In humans, malaria rapidly depletes blood sugar and causes anemia and vitamin deficiencies. These metabolic disturbances spark intense sugar cravings, prompting the infected person to consume more glucose, which in turn benefits the parasite’s survival and facilitates its uptake by feeding mosquitoes.

2 1

Chlorovirus ATCV‑1, a virus that normally infects algae, has been found in the human throat and is capable of impairing cognitive function. In laboratory mice, infection leads to marked deficits in learning and memory.

Human studies reveal that carriers of ATCV‑1 experience measurable drops in cognitive performance. Surprisingly, the virus can persist silently for years, with one U.S. study detecting it in 44 % of participants.

This “stupid virus” demonstrates that even seemingly harmless microorganisms can subtly erode mental acuity over long periods.

1 Influenza

Recent research suggests that the flu virus, as well as its vaccine, can make people more socially inclined. The hypothesis is that a socially active host increases the pathogen’s chances of hopping to new individuals within real‑world social networks.

Studies show that those who receive the flu vaccine display heightened sociability, mirroring the behavior of infected individuals who seek out contact with others. While the precise mechanisms remain unclear, the pattern aligns with other pathogens that subtly nudge hosts toward behaviors that boost transmission.

How 10 Parasites Pathogens Manipulate Minds

Across the spectrum—from tiny amoebae to elusive viruses—these ten organisms illustrate nature’s cunning ability to hijack human thoughts, cravings, and actions. Understanding their tactics not only satisfies curiosity but also informs public‑health strategies aimed at out‑smarting these microscopic masterminds.