The Worst Human Parasites

Human parasites are organisms that rely on humans to live yet don't offer anything positive to the people they infect. Some parasites can't live without a human host, while others are opportunistic, meaning they'd happily live elsewhere, but they make do if they find themselves in the body.

Here's a list of particularly nasty parasites that infect people and a description of how you get them and what they do. While any parasite picture probably makes you want to bathe in bleach, the images in this list are clinical rather than sensational.

Plasmodium and Malaria

Malaria merozoites ultimately rupture red blood cells, dispersing more parasites


There are about 200 million cases of malaria each year. While it's common knowledge that malaria is transmitted by mosquitoes, most people think it's a viral or bacterial disease. Malaria actually results from infection by a parasitic protozoan named Plasmodium. While the disease doesn't look as frightening as some parasitic infections, its fever and chills can progress to death. Treatments exist to reduce the risk, but there is no vaccine.

How You Get It

Malaria is carried by the Anopheles mosquito. When the female mosquito bites you—males don't bite—some Plasmodium enters the body through the mosquito's saliva. The single-celled organism multiplies inside red blood cells, eventually causing them to burst. The cycle is completed when a mosquito bites an infected host.

Tapeworm and Cysticercosis



Tapeworms are a type of flatworm. There are many different tapeworms and many different hosts for the parasites. When you ingest the eggs or larval form of some tapeworms, they attach to the lining of the gastrointestinal tract, grow, and mature to shed segments of themselves or eggs. Aside from depriving the body of some nutrients, this type of tapeworm infection is not a serious health risk.

However, if conditions aren't right for the larvae to mature, they form cysts. The cysts can migrate anywhere in the body, waiting for you to die and presumably be eaten by an animal that has a gut more suited to the worm. The cysts cause a disease called cysticercosis.

Infection is worse for some organs than others. If you get cysts in your brain, it can lead to death. Cysts in other organs can put pressure on the tissue and deprive it of nutrients, reducing function.

How You Get It

You can get tapeworms in many different ways. Eating snail larvae from improperly rinsed lettuce and watercress, undercooked pork or sushi as well as accidentally ingesting a flea or fecal matter, or drinking contaminated water are common routes of infection.

Filarial Worms and Elephantiasis

Micro-filarial worm under an electron microscope

David Spears FRPS FRMS / Getty Images

The World Health Organization estimates over 120 million people are infected with filarial worms, a type of roundworm. The worms can clog lymphatic vessels. One of the diseases they can cause is called elephantiasis, or the "Elephant Man Disease." The name refers to the massive swelling and tissue deformity that results when lymphatic fluid can't drain properly. The good news is that most people infected with filarial worms show few or no signs of infection.

How You Get It

Roundworm infections occur in many ways. The parasites can slip between skin cells when you are walking through damp grass. You can also drink them in your water, or they can enter through the bite of a mosquito.

Australian Paralysis Tick

Ticks are parasites that carry a variety of diseases
seraficus / Getty Images

Ticks are considered ectoparasites, meaning they do their parasitic dirty work on the outside of the body rather than internally. Their bite can transmit a number of nasty diseases, such as Lyme disease and rickettsia. Usually, though, it's not the tick itself that causes the problem.

The exception is the Australian paralysis tick, Ixodes holocyclus. This tick carries the usual assortment of diseases, but you can consider yourself lucky if you live long enough to get them. The paralysis tick secretes a neurotoxin that causes paralysis. If the toxin paralyzes the lungs, death from respiratory failure can result.

How You Get It

The good news is you only encounter this tick in Australia, probably while you're more worried about venomous snakes and spiders. The bad news is there is no antivenom for the tick's toxin. Also, some people are allergic to the tick's bite, so they have two ways to die.

Scabies Mite

A single Sarcoptes scabiei mite
Science Picture Co / Getty Images

The scabies mite (Sarcoptes scabiei) is a relative of the tick—both are arachnids, like spiders—but this parasite burrows into the skin rather than biting from the outside. The mite, its feces, and the irritation to skin produce red bumps and intense itching. While an infected person will be tempted to scratch his skin off, this is a bad idea because the resulting secondary infection can be serious.

People with weak immune systems or sensitivity to the mites can develop a condition called Norwegian scabies or crusted scabies. The skin becomes rigid and crusty from infection with millions of mites. Even if the infection is cured, the deformity remains.

How You Get It

This parasite is transmitted by contact with an infected person or his belongings. In other words, watch out for itchy people in schools and next to you on planes and trains.

Screwworm Fly and Myiasis

The maggots of the screwworm fly devour human flesh
Malte Mueller / Getty Images

The scientific name of the New World screwworm is Cochliomyia hominivorax. The "hominivorax" part of the name means "man-eating" and is a good description of what the larvae of this fly does. The female fly lays around 100 eggs in an open wound. Within a day, the eggs hatch into maggots that use their cutting jaws to burrow into the flesh, which they use as food. The maggots burrow through muscle, blood vessels, and nerves, growing the whole time.

If someone attempts to remove the larvae, they respond by digging deeper. Only about 8 percent of infected people die from the parasite, but they suffer the agony of literally being eaten alive, plus the tissue damage can result in secondary infections.

How You Get It

The screwworm used to be found in the United States, but today you need to visit Central or South America to encounter it.

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Your Citation
Helmenstine, Anne Marie, Ph.D. "The Worst Human Parasites." ThoughtCo, Aug. 29, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 29). The Worst Human Parasites. Retrieved from Helmenstine, Anne Marie, Ph.D. "The Worst Human Parasites." ThoughtCo. (accessed May 31, 2023).