Explore the 14 most horrifying human parasites that exist and learn how to protect yourself from them.

When it does enter a human nose, it finds nerve and brain tissue quite appetizing. It begins by attacking your olfactory bulbs, leading to a loss of your sense of smell and taste. Once it reaches your brain, your immune system reacts violently, and the cause of death is encephalitis, or brain inflammation.

How to avoid it: Try to stay clear of situations where warm water (like from springs or lakes) could enter your nose. Fortunately, it cannot be contracted from drinking water or through skin contact. Remember, it’s a rare event, so try not to get too unlucky. And if you use a neti pot to rinse your nose, boil the water first.

Let’s dive into the world of eyeball worms. Onchocerciasis is an illness caused by a nematode worm, transmitted through blackfly bites. These worms and flies thrive in tropical regions of Africa and South America, often near fast-flowing rivers.

The worms can travel throughout the human body, frequently making their way to the eyes. Once in the cornea, they can obstruct vision. If they block the eye's drainage system, they may cause increased eye pressure, or glaucoma, which can ultimately lead to blindness. The nodules formed by the worms can also be incredibly itchy and painful wherever they appear.

How to avoid it: Aside from avoiding areas where onchocerciasis is prevalent (which is hard if you live there), you can reduce the risk by using insect repellent and wearing clothing that protects against bug bites. Ivermectin is a medication that can eliminate the parasite, and local community programs often spray insecticides to reduce blackfly populations.

The parasites responsible for schistosomiasis are often called “blood flukes.” These microscopic flatworms spend part of their lifecycle inside freshwater snails. When people swim or wade through contaminated water, the flukes can burrow into the skin. From there, they travel to the bowel or bladder, where they can cause bleeding severe enough to result in anemia. The lesions they cause are also extremely painful.

How to avoid it: If you reside in areas where these flukes are common (such as Africa, the Middle East, the Caribbean, and parts of South America), avoid swimming or wading in fresh water. Chlorinated pools and seawater are safe, according to the CDC. Boiling water will kill the parasite. If infected, there is an antiparasitic drug called praziquantel that can eliminate it.

This worm has a unique journey through the human body. (Parasites sure are inventive!) Guinea worms lay their eggs in water, and the first stage of their life cycle takes place in tiny insect-like creatures that live in the water. If you drink contaminated water, the larvae can mature and grow inside you.

Things get even more unpleasant from here. The worms travel throughout your body, and if both male and female worms are present, they mate. The female then seeks out a location on your leg or foot and begins emerging about a year after the initial infection. The spot where the worm emerges becomes a blister that causes excruciating burning pain, and guinea worm and its associated complications are a leading cause of disability in regions where it’s prevalent. Dipping your foot in cool water may provide temporary relief, but does the worm swim out? No! It releases eggs into the water, continuing the cycle and infecting the next batch of bugs and people.

How to avoid it: The key to prevention is drinking safe, worm-free water. There is an ongoing global effort to eradicate guinea worm, and so far, only four cases have been reported in Chad this year. While rare, if you do contract the disease, please avoid putting your foot into local water sources. Reach out to your local guinea worm eradication organization (if you live in a region where guinea worm is common, there is one) for immediate treatment.

Tapeworm infections are usually pretty low-key. You eat undercooked meat containing tapeworm larvae, the tapeworms grow and mature in your intestines, and they hang around for years, feeding on whatever you consume. Think of them as a pet—inside your body.

(Just to clarify, the ‘snacks’ are metaphorical. Tapeworms actually absorb nutrients through their skin. They also latch onto your intestinal walls using hooks and suckers to make sure you can’t easily expel them.)

However, things can go wrong. If you're already malnourished, the worm's feeding might worsen your condition. (Though it probably won’t lead to stylish weight loss.) Additionally, a large number of tapeworms or excessively large ones can obstruct your digestive system.

The real danger arises if you accidentally ingest pig tapeworm eggs. Normally, humans ingest larvae in pork, and our internal tapeworms release eggs in our stool. Pigs ingest these eggs when they root around in filth, restarting the cycle. But if a human consumes the eggs (perhaps through contaminated water), the larvae can travel outside the intestines and form cysts in muscles or, in more serious cases, the brain. This can lead to seizures. Tapeworm cysticercosis (cysts in body tissues) is a leading cause of epilepsy in areas where it’s common.

How to avoid it: Make sure to cook your pork thoroughly and purchase it from regulated sources (pork sold in the US is typically safe). While other animals can harbor tapeworm larvae, the most significant risk for humans comes from pig tapeworms. In areas where pigs roam freely, practice good hygiene and avoid drinking contaminated water. Boiling water will kill the eggs.

Elephantiasis, a condition caused by filarial worms, leads to swelling in parts of the body like the legs and genitals. This swelling results in thickened skin and fluid buildup. The images are not for the faint-hearted, so I’ll leave it to you to search for them.

The filarial worms are tiny and spread through mosquito bites. Once inside, they reside in our lymphatic vessels, where they reproduce, releasing larvae into the bloodstream. These larvae are then ingested by the next mosquito that bites the infected person, completing the cycle. The real damage is done when the worms block the lymphatic vessels, causing the fluid buildup characteristic of this disease.

How to avoid it: If you live in an area where these worms are prevalent, taking anti-parasitic medications once a year can help kill the worms. Additionally, controlling mosquito populations is essential in preventing this condition.

Previously, we mentioned the Australian paralysis tick while discussing the alpha-gal allergy, where a tick bite can trigger a lifelong meat allergy. While the Lone Star tick often takes center stage for this, the Australian paralysis tick is also capable of carrying the disease.

However, that’s not why it made our list. This tick has the ability to paralyze you. A bite from it can lead to intense itching and a painful lump at the bite site. As the days pass, you may begin to notice symptoms like weakness in the arms and legs, difficulty walking, or even facial drooping. The only treatment for paralysis is removing the tick, but if it stays attached and symptoms progress, you could experience difficulty breathing and potentially die.

How to avoid it: The best way to avoid tick bites is to use repellent and thoroughly check your body for ticks after spending time in areas where ticks are common. If you find a tick, remove it right away, even if you don’t feel any immediate symptoms. While this particular tick is found on Australia’s east coast, paralysis from ticks like the Lone Star tick has been reported in the US as well.

Most maggots feed on decaying tissue. They’re drawn to meat, and when they find an open wound, they only feed on the dead tissue. In fact, they’re often used in medical settings to clean wounds.

But screwworms are a different story: they feed on living tissue, burrowing deep into it with only their breathing tubes exposed. They can infest any open wound or body opening with mucous membranes, including nostrils, eyes, ears, and genital areas. As they burrow, the wound becomes painful and emits a foul smell.

How to avoid it: To prevent screwworms, ensure your wounds stay clean. If infected, treatment includes medication to eliminate the worms. While screwworms are more common in livestock, they can occasionally affect humans. They have been eradicated in the US but may make occasional reappearances. If you encounter a screwworm infection, report it to your local agricultural or public health authority.

Since we’re discussing maggots that burrow under the skin, we can’t skip over the botfly. These little creatures take a unique approach: they target mosquitoes in midair, lay their eggs on them, and then let the mosquitoes pass the eggs on to animals or humans when they bite. Once the eggs land in the bite, the larvae crawl in and start growing. If you’re a fan of pimple-popping videos, botfly removal videos might be right up your alley.

How to avoid it: To protect yourself, use mosquito and tick protection, as they can carry botfly eggs. If you notice a painful lump that seems to move when you shower or cover it up, seek medical attention to have it safely removed.

Pinworms hold the title for the most common worm infection in the United States. These tiny parasites reside in the large intestine and have a particularly unpleasant habit: at night, they crawl out to the anus to lay their eggs. The eggs cause itching, and when you scratch the affected area, the eggs can get under your nails. From there, they can be transferred to your mouth or someone else's.

How to avoid it: Make sure to wash your hands after scratching—it's a simple yet effective step. Pinworm infections are most common in children, who may not have mastered basic hygiene practices. The eggs can also spread via clothing, bedding, and other objects; they’re even small enough to be inhaled. Regular handwashing can go a long way in prevention.

You might have heard of scabies, but did you know it’s basically the same as mange? Fun fact! Scabies is a highly contagious skin condition caused by mites—tiny relatives of ticks and spiders. The itching may not start until a couple of months after infection, but the mites can be spread during that time. Scabies spreads through prolonged skin-to-skin contact, so it’s commonly associated with sexual activity and can appear in areas like the groin (where the mites love to nest), though household contact or close proximity with anyone who has it can also transmit it.

How to avoid it: Avoid direct skin contact with anyone who has scabies. Prescription treatments are available to eliminate the mites once you’re diagnosed.

You’ve probably heard of head lice, which are unpleasant but mostly harmless. Body lice, however, are a different matter—they can spread the deadly disease known as typhus.

Body lice and typhus typically affect those living in overcrowded conditions, especially when regular access to showers or fresh clothing is limited. During World War II, body lice became a widespread problem among soldiers, civilians in war-torn villages without proper sanitation, and prisoners in concentration camps. Anne Frank tragically succumbed to typhus.

How to avoid it: Ensure you bathe and change clothes at least once a week. Also, the CDC recommends avoiding contact with flying squirrels.

Malaria is caused by a protozoan parasite transmitted by mosquitoes. Unlike viruses or bacteria, this tiny organism reproduces within red blood cells, triggering a cycle of fevers that occur every other day or intermittently. Historically, doctors classified these fever patterns as 'tertian' or 'quartan,' terms linked to malaria before the parasite was identified. There are five main types of malaria that affect humans, with some variants being more lethal than others. Malaria remains one of the primary causes of childhood mortality in regions of Africa where it is prevalent.

Prevention: When traveling to areas where malaria is common, it's important to take preventive medications as prescribed. The CDC offers detailed guidance for travelers on their website. Additionally, protecting yourself from mosquito bites is crucial—use repellent, sleep under bed nets, ensure windows are screened, and wear protective clothing to minimize exposure.

If you're a pet owner, you may already know that heartworms are a risk for dogs and cats (although dogs are more prone to them). However, humans can occasionally be affected by this parasite, which is why it's included in this list.

Heartworms are filarial worms, similar to those responsible for causing elephantiasis. In dogs, they typically inhabit the heart and pulmonary blood vessels. In humans, the worms are unable to fully develop and we are not contagious. Nonetheless, the dying worms can lead to lesions, and symptoms such as coughing up blood, fever, and chest pain may occur. In severe cases, surgical removal of the dead worms may be necessary.

How to prevent it: Protect yourself and your pets from mosquito bites. While you're thinking about it, make sure your dog stays up to date with their heartworm prevention treatment.