The phrase “flesh-eating” is nightmarish enough to stop anyone dead in their tracks, conjuring up images of flesh falling off of the bone, deep black necrosis, or perhaps a zombie apocalypse in which the undead are biting flesh from bone. Be it flesh-eating zombies, flesh-eating animals, or, even more terrifying, flesh-eating microscopic organisms, the idea of being eaten alive is downright terrifying, and when it comes to the latter, it’s more frightening to think of something you can’t see that slowly eats away at you.

The world of pathogens is a strange world indeed, and there are some pretty nasty critters out there that would love nothing more than to take a bite out of your flesh—and sometimes bone. Some pathogens hijack the minds of their human hosts, controlling their behavior, their thought patterns, and even their actions. Others lodge themselves inside you and consume you from the inside out. Here are ten nightmarish flesh-eating pathogens and their destructive ways.

10 Necrotizing Fasciitis

Necrotizing fasciitis deserves an honorable mention here at the top of the list, as it’s the result of several infections on this list. Necrotizing fasciitis is caused by the presence of pathogens which cause the skin to rot, especially when they become entrenched into the inner layers of the flesh. If it isn’t treated right away, necrotizing fasciitis can kill you, with anywhere from 25 to 30 percent of the cases being fatal.

Necrotizing fasciitis has earned the moniker “flesh-eating disease,” which is quite the terrifying name. While the infections that cause it vary, the pathogens don’t literally eat the flesh but rather excrete toxins that destroy tissue, causing the condition.^[1] Drug and alcohol users as well as diabetics and those who are immunocompromised are particularly at risk for necrotizing fasciitis.

9 Vibrio Vulnificus

Vibrio vulnificus is often found in a place that’s a perfect climate for the bacterium to grow and also possesses the large bodies of water which it likes to flourish in: Florida. A person can become infected either by going into the water with an open cut or wound or consuming undercooked or uncooked seafood, as it lives and thrives in salt water. Vibriosis, the disease spawned from the Vibrio genus of bacteria, can be fatal. It should be noted, as alluded to above, that V. vulnificus causes the flesh to rot rather than eating it, but that difference is quite irrelevant when you see chunks of your arm missing.

Even more terrifying is that V. vulnificus not only attacks the surface flesh but can bury itself and get underneath the top layer of tissue. It can even seep in and cause the internal organs to rot away and even eventually shut down. Sometimes, people lose limbs to the disease.^[2] Again, the immunocompromised are much more likely to contract this flesh-rotting bacterium, as well as people with liver problems. The Centers for Disease Control in the United States suggests not eating raw or undercooked fish if you’re trying not to catch this, and 80 percent of cases come between May and October, when the water is warmer, so these are the best times to avoid the water to avoid the disease.

8 Donovanosis

Donovanosis, also known as granuloma inguinale, is a disease that comes from the bacterium Klebsiella granulomatis. It’s relatively new on the scene, and it may very well be the scariest thing on this list. Why? Because it’s an STD. That’s right, donovanosis is a flesh-eating STD.^[3] The bacteria can destroy the flesh in, around, or on the genitals and can do more damage if they spread to other parts of the body. The damage to the exterior flesh can cause large, red, vascular lesions, and these holes in your flesh bleed.

This disease typically makes its initial home at sites in and around the pelvis but can work its way up inside of you and get into your organs, damaging them, too. If that wasn’t bad enough, in extreme cases, Klebsiella granulomatis can get into your bones as well if not treated promptly. Fortunately, the treatment is rather simple, with broad-spectrum or targeted antibiotics. But imagine waking up one day to find your genitals and the surrounding areas slowly rotting—even if it’s treatable, this is a day that nobody wants to see.

7 Pseudomonas Aeruginosa

Pseudomonas aeruginosa is a bacterium that grows pretty much everywhere and on pretty much everything, though it doesn’t always infect us—instead, it waits for an opportune opening, such as a cut or scrape, and when the conditions are just right, it will seep into the wound and multiply. P. aeruginosa can cause necrotizing fasciitis. The good news is that healthy people generally have a very low risk of becoming infected by this particular bacterium, but as is often the case, those who have immune system deficiencies can experience problems with it.^[4]

If left untreated, Pseudomonas aeruginosa can reach the internal organs and cause death. If left alone, it will eat all the way down to the bone. As with other Pseudomonas species, some of the more severe cases invole infections in the blood or the lungs. What’s really the most terrifying thing about Pseudomonas is the fact that many strains of it have developed antibiotic resistance and are quite difficult to treat, meaning that P. aeruginosa could eat away at you while doctors attempt to find an antibiotic regimen that will work to eliminate the bacteria.

6 Staphylococcus Aureus

Yes, as freaky as it sounds, the common staph infection can become necrotic and flesh-eating. Methicillin-resistant Staphylococcus aureus (MRSA) is Staph aureus with additional antibiotic resistance, making infections rather difficult for modern medicine to treat. Fortunately, not all cases of staph and MRSA turn into flesh-eating necrosis, but doctors can’t actually tell which strains and cases will eventually turn into a condition that eats away at the flesh, so it’s better to treat all situations involving these bacteria as if they were about to become necrotic.^[5]Staph aureus and its antibiotic-resistant counterpart both present with a nasty, thick pus which can get deep into the skin, working its way into muscles and ultimately into the bloodstream—once these infections seep into the blood, a powerful and brutal sickness ensues, and death is a likely possibility. On top of that, if the condition brings about necrotizing fasciitis, you’ll have some serious decay and blackening of the tissue layers as the excreted toxins from the bacteria wreck havoc.

5 Naegleria Fowleri

Naegleria fowleri is an amoeba that severely affects the brain on the rare occasion that it inhabits a human host—these dangerous critters love to hang out and reproduce in fresh water, making them ample under the right climate conditions, though they need to travel up the nose and into the brain in order to “hijack” your mind. How do they do that? By reproducing and spreading quickly and then slowly eating away at your brain, that’s how.

Under certain conditions, the amoeba just forms into a cyst that’s inactive and doesn’t do much of anything, but when conditions become right, they become active and begin to feed. The Centers for Disease Control says about the mechanism of death from this particular amoeba: “The infection destroys brain tissue causing swelling and death.”^[6] This is definitely not a fun pathogen to catch by any stretch of the imagination.

4 Clostridium Perfringens

Clostridium perfringens is another nasty one. It can be found in soil, water, and the human intestinal tract. This bacteria can cause a specific kind of gangrene called gas gangrene, and it definitely does not at all look very fun. The toxins which are excreted by the bacteria travel through veins, blood vessels, and other bodily systems, and they kill off the flesh inside of the body by poisoning it.^[7]

This causes massive distention, huge pockets of bloating gas that come from the bacteria feeding and releasing of gasses inside the body. Massive bulbs of purple and red swelling show up on the outside of the body as the internal gasses protrude and push outward.

3 Streptococcus Pyogenes

Possibly the best-known cause of necrotizing fasciitis is Streptococcus pyogenes, also referred to as “Group A Streptococcus.” This bacterium also causes strep throat, which many people, even in the developed world where antibacterial everything is prevalent, end up catching at one point, often as children. Like the others on this list, Streptococcus pyogenes produces toxic byproducts which cut through tissue layers of fat, muscle, and skin like butter. This infection starts off rather mild, most commonly presenting as a sore throat, but it can progress and eat away at the tissue of the infected and even cause toxic shock syndrome, a condition that comes with headache, nausea, vomiting, and much more and is also definitely no fun to get.

While most cases of strep throat don’t turn into “the flesh-eating disease,” some of them do, and the effects can be both long-term and devastating.^[8] This happens when the body’s immune response isn’t quick or powerful enough to fight off the invasive strep, and the results can be deadly. Yes, the common strep throat can actually eat your flesh until you die.

2 E. Coli

Escherichia coli is a relatively well-known bacterium which is often responsible for the common food poisoning we all miserably endure every so often. This is the usual E. coli infection, and it’s nothing to shake a stick at, but sometimes, E. coli can go above and beyond the usual case of what we know of as “food poisoning” and become catastrophically worse. As with most bacteria, the body’s immune system as well as antibiotics have fought off the invaders for a very long time, causing variations in the strains which possess radically different traits and can have radically different outcomes upon infection. Some strains of E. coli can cause the death and decay of the skin cells also as they slowly eat away at your flesh.

These strains prey upon people with limited or suppressed immune systems, which can’t fight off the infection—all cases in some studies have been fatal. A specific gene called the cnf1 toxin gene is possessed by the flesh-eating variations of E. coli, which causes them to emit a toxin which destroys flesh like an acid. Animal studies have shown that the presence of this gene in the bacteria causes the toxin to be excreted when animals are infected as well. These E. coli strains are the stuff that nightmares are made of.^[9]

1 Mycobacterium Ulcerans

Mycobacterium ulcerans is a bacterium which is the cause of a disease called buruli ulcer, which causes, you guessed it, ulcers to appear within the skin. But much more than this, buruli ulcer is also a flesh-eating condition. Prolonged infection will cause ulcers on the arm and legs typically, and the chunks of flesh which have been eaten away will usually worsen over time. Only ten percent of cases present with flesh-eating ulcers in other parts of the body outside of the limbs, but that’s still a scary ten percent. As the bacteria spread, they produce a toxin called mycolactone, which is responsible for the destruction of flesh in patients who’ve contracted the bacterial infection and allowed it to progress to the disease of buruli ulcer.

Even more terrifying than having this bacterium secrete toxins which eat away at your flesh while you wait helplessly for it to go away, or for your treatments to work, is that beyond just skin, ligament, muscle, and other soft tissues, the disease can eventually spread down to the bone and begin to eat away at that, too.^[10] And what’s even more nightmarish than that? They have no idea how this bacterium spreads. Very little is known about how M. ulcerans transmits from one person to the next, so we don’t really know what any of us can do to avoid becoming its next target—yet. Hopefully someday, science will develop methods to eradicate these bacteria as well as new and unique treatments, but for now, we’re stuck with limited knowledge and treatment options.

I like to write about the dark, strange, macabre, and unusual.

There are many common microorganisms that sometimes make their way into the local and national news. These may include microorganisms that cause food poisoning, fever, pneumonia, and more. However, these common pathogens, usually residing unnoticed on the different parts of our body, can actually turn against us and begin eating our flesh! This ranges from killing surrounding tissues to actually ingesting flesh and brain matter.

10 Streptococcus Pyogenes

Streptococcus pyogenes, also known as group A streptococcus (GAS), is a normal flora of the human body. Under ordinary circumstances, it resides on our skin and other parts of the body. Since it is naturally pathogenic, it can sometimes cause diseases such as strep throat and scarlet fever.

In more severe cases, this bacterium can actually eat our flesh in what is called necrotizing fasciitis. In 1999, the CDC reported 600 cases of necrotizing fasciitis by S. pyogenes. The bacterium releases toxins and enzymes that directly attack and kill surrounding body cells.

If left untreated, it can eat away large parts of the body and cause death. Unfortunately, a newly discovered strain of S. pyogenes called emm89 is becoming prevalent and is a potent, causative agent of necrotizing fasciitis.

It is important to know that flesh-eating disease is only common to those with immunocompromised bodies, such as diabetics.

9 Apophysomyces

Soil harbors an extreme number of microorganisms, including a fungus called Apophysomyces. Although infection from this fungus is quite rare, it can still cause a flesh-eating disease called mucormycosis.

Mucormycosis is not unique to Apophysomyces. Other fungi belonging to the order Mucorales, such as Mucor and Rhizopus, can also cause it. To be pathogenic, it must be delivered deep into the flesh.

One unfortunate incident with this disease occurred when a tornado hit Joplin, Missouri, in 2011. Thirteen injured survivors soon found themselves infected by the fungus. As the fungus grew, it invaded nearby tissues using its mycelia, damaging blood vessels and restricting blood flow.

Starved of blood and nutrients, the tissue began to die. As scientists discovered, the sheer force of the tornado delivered the fungus from the soil to deep inside the wounds of the victims in an unusual and extremely unlucky event.

8 Leishmania

Even a normal fly bite can progress to a flesh-eating disease in certain circumstances. Leishmania is a parasite common in the tropics and subtropics, including countries in southern Europe. Its life cycle needs both a human and a sand fly host.

After living in a sand fly and maturing to a certain degree, Leishmania is transferred to the human body through a fly bite. The parasite then matures more in the human body and invades nearby cells. The affected cells burst and die after serving as residences for the parasite.

Flesh eating almost always happens around the bite and can enlarge significantly if left untreated. On rare occasions, the parasite can become systemic, migrate to other parts of the body, and begin to eat them, too.

There are two types of leishmaniasis: cutaneous and visceral. The former type manifests as raw, erupted skin, while the latter affects the internal organs. Right now, war-torn Syria is experiencing an outbreak of leishmaniasis. Travelers to the Amazon and Africa are also susceptible to this infection.

7 Aeromonas Hydrophila

Aeromonas hydrophila is common in fresh and brackish waters, such as estuaries. It can also contaminate drinking water when purifying equipment malfunctions or is suboptimal. Travelers are usually infected by this bacterium, which manifests as diarrhea (aka traveler’s diarrhea).

The bacterium can also contaminate food products when water used to clean the facilities or products is contaminated. Since Aeromonas hydrophila is resistant to cold temperatures, storing the contaminated foods in the refrigerator may not effectively eliminate the contamination and may later affect people who eat the products.

Though it is rare, a person can be infected by this bacterium and experience flesh-eating disease when wounded parts of the body come in contact with contaminated water, as in the case of a woman from Georgia in 2012. The bacterium entered her body through a wound acquired from a zip line accident.

Amputation and removal of some affected internal organs are common treatment options to stop the spread of the disease. So try not to dip in the water if you have open wounds.

6 Bacteroides Fragilis

Bacteroides fragilis thrives in the oxygen-deprived environment of the human gut, especially in the colon. It is a normal resident of our body and actually aids in digestion. This bacterium also prevents the invasion of other pathogenic microbes by competing for space and nutrients. Since the bacterium is well-adapted to the gut environment, foreign and invading microbes are at a disadvantage most of the time.

Ironically, when this commonly friendly bacterium leaves the gut, usually during surgery or a traumatic accident, it can cause severe necrotizing (flesh-eating) infections. These infections are usually characterized by excessive puss and edema in the surrounding tissues.

Unfortunately, treatment of a B. fragilis infection is moderately cumbersome. The bacterium is capable of destroying penicillins and its relatives by producing enzymes that break down the antibiotic. Usually, higher forms of antibiotics, such as carbapenems, are needed. However, frequent use of these higher-generation antibiotics may also lead to increased antibiotic resistance.

5 Clostridium Perfringes

Clostridium perfringes, a cousin to the Clostridium botulinum bacterium that causes botulism, is another common pathogenic species of the Clostridium genera. It is commonly found in soil and the human gut. It is also common in raw meat and poultry products.

When these products are eaten uncooked, the bacterium may cause food poisoning. In extreme cases, C. perfringes can cause necrotizing fasciitis, just like Streptococcus pyogenes, Aeromonas hydrophila, and Bacteroides fragilis.

Clostridium perfringes may eat the surface of the skin or the deeper parts of the muscle, usually leading to amputation. In addition, C. perfringes may cause gas gangrene.

Gas gangrene is a serious infection and involves the muscle instead of just the skin. It is characterized by gas production on the dying muscle tissues. Usually, infection by this bacterium happens when a wound comes in contact with contaminated soil.

This scenario is common in the battlefield, where wounds may be left open and exposed to dirt. Common treatment options are skin grafting and amputation.

4 Klebsiella Pneumoniae

Klebsiella pneumoniae is considered a normal resident of the human body, usually in the gut and in the nasopharynx areas. Considered to be one of the most antibiotic-resistant microorganisms, Klebsiella pneumoniae is associated with numerous diseases, such as pneumonia, bloodstream infections, and meningitis. Infection by this pathogen is common in hospital settings, where the bacterium resides in catheters and breathing apparatus.

On rare occasions, Klebsiella pneumoniae may cause flesh-eating disease. The first case in North America was documented in an article published in the Western Journal of Emerging Diseases. An elderly Filipino woman living in California presented with muscle necrosis caused by K. pneumoniae. According to the paper, such cases are normally found in southern Asia where the bacterium is prevalent.

Infections from Klebsiella pneumoniae are particularly hard to treat because they are resistant to a wide variety of antibiotics. In fact, it is well-known that K. pneumoniae is resistant to all penicillins and increasingly resistant to higher generations of drugs as well.

3 Vibrio Vulnificus

Vibrio species are usually found in marine environments because they require salty water for growth. Both Vibrio cholerae, which causes cholera, and Vibrio vulnificus are members of this genus.

Vibrio vulnificus is usually contracted from marine foods, such as crabs, and may cause diarrhea and vomiting when the bacteria is ingested. On rare occasions, susceptible individuals may also develop liver disease. When open wounds are exposed to seawater contaminated with V. vulnificus, necrotizing infections may occur.

Necrotizing infections caused by V. vulnificus are common to areas near the coast, like US states that border the Gulf of Mexico. In 2015, several cases of flesh-eating disease caused by V. vulnificus were recorded. In fact, Florida has as many as nine deaths per year due to this bacterium.

Infection is most common in May and October. To prevent necrotizing fasciitis caused by Vibrio vulnificus, it is better to keep open wounds away from saltwater environments, especially during the warm season when the bacterium is prevalent.

2 Staphylococcus Aureus

Staphylococcus aureus (aka Staph) is a common pathogen that may reside transiently on the human skin and other parts of the body. According to the CDC, 30 percent of the global population carries the bacterium in their nasal region. In most cases, they are nonpathogenic events.

Staph is associated with bloodstream infections and infection of the heart valves (aka endocarditis) when introduced to the bloodstream via open wounds or surgical operations. When ingested, it may cause food poisoning when the bacterium releases toxins. Cooking may effectively kill it, but the toxins remain undamaged.

In extreme cases, Staphylococcus aureus may cause necrotizing fasciitis. This is common with patients who have underlying illnesses such as diabetes and hypertension. With impaired immune defenses, the bacterium can easily invade and kill body tissues. Treatment of these infections may be difficult because many of these cases are caused by methicillin-resistant Staphylococcus aureus (MRSA), which is tolerant to many available antibiotics.

1 Naegleria Fowleri

The next time you dive into a freshwater environment, you may want to cover your nose as a safety precaution. Common to lakes, rivers, and other freshwater environments, Naegleria fowleri is an amoeba that actually ingests brain matter.

Upon entering the nose, the amoeba travels through the olfactory nerves until it reaches the brain, where it feeds. Although its usual diet includes bacteria, that particular type of food is severely lacking in our brains. So Naegleria fowleri switches to actually eating our brains instead.

Although natural bodies of water are common sources of this amoeba, contaminated pools or tap water may also harbor this zombie pathogen. It is important to know that contaminated water must pass through the nose for infection to occur. Infection cannot happen if the amoeba is ingested through the mouth or any other body openings.

Infection by Naegleria fowleri is rare but almost always leads to death when it occurs. Currently, there is no standard drug used to treat this type of infection, which usually requires surgery as a last resort. To avoid infection by this brain-eating amoeba, avoid untreated and warm freshwater environments. Properly chlorinated pool and tap water is generally safe.

Matthew is a premedical student majoring in microbiology.

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.

It is weird to think that diseases can be used to cure other diseases. However, it is true. For centuries, scientists have figured out ways of using deadly, pathoegenic bacteria, viruses, and protozoa to cure deadly diseases caused by other pathogens.

It may seem counterintuitive to cure one illness with another, but it has worked time and time again. And it’s not always that scary. Other microorganisms, specifically viruses and bacteria that are not necessarily dangerous to humans, have also been utilized to treat lethal diseases.

10 Malaria

Syphilis was incurable throughout most of history and often led to death within four years. The worst form is neurosyphilis, which is infection of the nervous system by syphilis and often the final stage of the disease. Neurosyphilis is accompanied by blindness, madness, paralysis, and, subsequently, death. Most syphilis patients were confined to asylums until they died.

Austrian psychiatrist Dr. Julius Wagner-Jauregg started developing a treatment for syphilis in the 1880s. He turned to pyrotherapy—the artificial induction of a fever, in this case by the introduction of a more manageable pathogen. The high fever caused by the introduced infection kills off the incurable disease, and the curable disease is treated thereafter.

Dr. Wagner-Jauregg unsuccessfully tried using tuberculosis antigen as well as the typhus and typhoid vaccines to cure syphilis. However, he got his break in June 1917, when a wounded soldier suffering from malaria was sent to the psychiatric ward of the hospital where he worked. This was clearly an error because the soldier had no mental issues. But the doctor seized the opportunity to work on his pyrotherapy treatment for syphilis.

Dr. Wagner-Jauregg extracted blood from the soldier and injected it into nine people suffering from advanced syphilis. The malaria-causing Plasmodium protozoa caused a serious fever that killed off the syphilis-causing Treponema pallidum bacteria. Dr. Wagner-Jauregg went on to cure the six survivors—who now had malaria—with quinine.

Dr. Wagner-Jauregg published his findings in 1918. He noted that syphilis is killed when the body maintains a temperature of 41 degrees Celsius (106 °F) for six hours. His treatment soon became the method of choice for syphilis. However, it had its downsides, even though it was considered successful.

Syphilis patients often suffered from complications when they were injected with a different blood type. They also inherited the blood diseases of their donors. The deadly malaria strain used at the time could also cause anemia and kidney failure. Doctors later switched to the less deadly Plasmodium vivax strain. This method of treatment was abandoned after the advent of antibiotics.^[1]     

9 HIV

It is especially surprising that one of the world’s worst diseases can be used to treat other deadly ailments. Scientists have discovered a way to use HIV to cure leukodystrophy and Wiskott-Aldrich syndrome, two deadly diseases that often affect children.

To be clear, we do not use HIV itself but viral vectors created in part from HIV. A viral vector is used to deliver genetic material into cells, as in gene therapy. In 2010, a team of Italian doctors led by Dr. Luigi Naldini injected 16 children with HIV-based viral vectors. Six had Wiskott-Aldrich syndrome, while the other ten suffered from leukodystrophy.

Three years later, they observed that six of the children were slowly recovering from the diseases. Three suffered from Wiskott-Aldrich syndrome, and the other three suffered from leukodystrophy.^[2] The other ten were also showing some signs of recovery. The procedure is inconclusive because it is still undergoing clinical trials.

8 Cancer

CRISPR (pronounced “crisper”) means “Clustered Regularly Interspaced Short Palindromic Repeats.” It is used in the CRISPR-Cas9 gene editing technology that allows scientists edit the DNA in cells.

While scientists concentrate on using CRISPR to modify unfavorable genes, researchers at the Rutgers Cancer Institute of New Jersey are trying to use it to cure cancer. CRISPR works for cancer treatment because cancerous cells roaming in the body tend to circulate back to the tumor they originated from.

Using CRISPR technology, researchers at the Rutgers Cancer Institute injected these cancerous cells with the cancer-killing S-TRAIL protein. The S-TRAIL-containing cancerous cells killed other cancerous cells in the tumors when they entered them. Then they essentially self-destructed thereafter. The technology has not been tested on humans, though, and experiments have been confined to mice.^[4]

7 Cowpox

The cowpox virus was used to create the first vaccine for smallpox, a lethal virus that infected millions of people until it was declared eradicated in 1977. The cowpox vaccine is the reason vaccines are called vaccines. The name was derived from vaccinus, the Latin word for “cow.”

While we credit England’s Dr. Edward Jenner for creating the first smallpox vaccine, we know ancient Chinese and Middle Easterners deliberately infected themselves with cowpox to make themselves immune to the virus for centuries.

Dr. Jenner created the smallpox vaccine after observing that milkmaids never contracted smallpox. He later realized this was because they had been infected by cowpox—a closely related virus—from the cows they milked. Dr. Jenner proved his theory right in 1796, when he deliberately infected eight-year-old James Phipps with cowpox.

Dr. Jenner infected Phipps with smallpox a month and a half after infecting him with cowpox. Phipps never developed smallpox, indicating that he was immune to the virus. Dr. Jenner later published his findings. The smallpox vaccine which later eradicated the disease was derived from the vaccinia virus, another closely related virus.^[4]

6 Poliovirus

Poliovirus causes polio, which used to be one of the deadliest diseases out there. Fortunately, it is on the brink of extinction today. Only 22 incidents of polio were reported in 2017, which is a far cry from the 350,000 cases reported in 1988.

Interestingly, scientists are developing a method of using the once-deadly disease to cure glioblastoma (GBM), a rare but deadly and highly aggressive form of brain cancer. Glioblastoma is treated with surgery, radiation, and chemotherapy. However, it often returns and kills the patient within about a year.

The poliovirus therapy was developed by researchers from Duke Cancer Institute, Durham, North Carolina. They genetically modified the poliovirus to create a new virus called PVSRIPO, which is injected right into the brain tumors caused by glioblastoma.

A clinical trial conducted on 61 glioblastoma patients indicated a 21-percent survival rate. That seems small until we realize that patients given the standard treatments have a survival rate of just four percent.

While PVSRIPO looks promising, it could cause some unfavorable side effects depending on the location of the tumor in the brain.^[5]

5 Bacteriophage Therapy

In 2015, 69-year-old Tom Patterson was diagnosed with pancreatitis (inflammation of the pancreas) while visiting Egypt with his wife. Conventional treatment did not work, and he was later flown to Frankfurt, Germany. Doctors drained the fluid around his pancreas to discover he was infected with drug-resistant Acinetobacter baumannii bacteria.

Patterson was later flown to Thornton Hospital, San Diego, California, where a drain was inserted around his pancreas to control the dripping. Unfortunately, the drain slipped, and the fluid leaked into his abdomen and bloodstream. Patterson soon started experiencing high fever, serious pains, and breathing difficulties. He also fell into a coma that lasted for about two months.

Doctors settled for bacteriophage therapy as a last-ditch effort to save his life. Unlike what the name suggests, bacteriophages are viruses and not bacteria. The name means “bacteria eater” and refers to a distinct class of viruses that attack bacteria. Every bacterium has a bacteriophage that has evolved to use it to replicate.

Bacteriophage therapy refers to the use of these bacteria-attacking viruses to cure bacterial infections. It was the go-to method of dealing with deadly bacteria until antibiotics came along. However, its results are not scientifically proven.

Nevertheless, the therapy worked, and Patterson was slowly recovering from the coma—until the A. baumannii bacteria mutated and developed resistance against the virus. Doctors solved this problem by passing a newer strain of the virus into Patterson’s body until he was finally cured.^[6]

4 Maraba Virus

Scientists have always known that the Maraba virus (aka MG1 virus) attacks and destroys cancer cells. However, scientists at the Ottawa Hospital and the University of Ottawa have discovered that the Maraba virus also attacks and destroys HIV-infected cells.

HIV works by infecting and rapidly multiplying in the immune system cells of its hosts. However, some HIV-infected cells turn dormant after some time, while others continue to reproduce.

Doctors often administer antiretroviral drugs to suppress HIV. However, the drugs only work on active HIV-infected cells and have no effect on the dormant cells. The dormant cells kick into action and begin to rapidly reproduce when the patient stops taking the antiretroviral drugs.

Lab tests have proven that the Maraba virus will destroy dormant HIV-infected cells, indicating a possible cure for HIV. However, the procedure is considered inconclusive since tests have only been conducted in the lab, and this method has not been tested on animals or humans.^[7]
 

3 Coley’s Toxin Treatment

Coley’s toxin treatment involves use of bacteria to treat cancer. The procedure is named after William Coley, a New York bone surgeon who developed it in the 1890s. Coley invented the treatment after observing that patients who get infected with bacterial diseases while recovering from cancer surgeries were often better off than uninfected patients.

Coley believed this happened because the bacterial infection strengthened the patient’s immune system. So he began injecting live bacteria into his cancer patients. He later switched to using dead bacteria, given that live bacteria could still cause deadly infections.

Scientists do not agree on how the process works. Some think the injected bacteria strengthen the immune system against cancer cells. Others think the bacteria actually encourage the production of either a protein called interleukin 12 (IL12) or tumor necrosis factor (TNF) proteins that fight cancerous cells. Another group think it’s high fever that kills the cancerous cells, just like the pyrotherapy procedure we mentioned earlier.

Nevertheless, Coley’s toxin treatment had mixed results. It worked with some patients but did not work with others. However, it was widely used until the early 1950s, when it was displaced by other cancer treatments like chemotherapy. An improved version utilizing genetically modified bacteria is still used today.^[8]

2 Predatory Bacteria

Predatory bacteria are those that attack and eat other microorganisms. Predatory bacteria work by attacking and breaching the walls of enemy bacteria cells. Once inside, it eats the bacterium’s innards before reproducing and leaving to attack other, similar bacteria cells.

Scientists are already working on using predatory bacteria to treat other bacterial infections, especially superbugs that have become immune to regular antibiotics.

In November 2016, the BBC reported that scientists at Imperial College London and the University of Nottingham had successfully used the Bdellovibrio bacteriovorus predatory bacteria to kill Shigella, a deadly genus of bacteria that causes food poisoning and kills over a million people a year.

Scientists observed Shigella populations reduce by 4,000 times after exposure to B. bacteriovorus in the lab. Another test in fish larvae saw the survival rate of the larvae infected with Shigella increase from 25 percent to 60 percent. Scientists plan on testing B. bacteriovorus on other deadly human bacteria, including Salmonella and E. coli.^[9]  

1 CAR-T Therapy

T-cells play a vital role in the body’s immune system. Recently, scientists have developed a method of using the T-cells to create chimeric antigen receptor T-cell therapy (CAR-T therapy), an anticancer treatment.

CAR-T therapy works by extracting the natural T-cells in the body and programming them with chimeric antigen receptors, which greatly improve their ability to detect, bind to, and destroy cancerous cells. The genetically modified T-cells are tailored to target the specific cancer affecting the patient, making them the perfect cancer cell assassins.

However, CAR-T therapy is only used as an option of last resort because it can cause a myriad of side effects, including brain inflammation. The process is also time-consuming, since the T-cells need to be tailored to the patient. The entire procedure could take four months.^[10]