A parasite is an organism that lives in another organism, called the host, and often harms it. It depends on its host for survival.

Without a host, a parasite cannot live, grow and multiply. For this reason, it rarely kills the host, but it can spread diseases, and some of these can be fatal.

Parasites, unlike predators, are usually much smaller than their host and they reproduce at a faster rate.

Parasites range from microscopic in size to over 30 meters in length.Share on Pinterest
Parasites range from microscopic in size to over 30 meters in length.

A parasite is an organism that lives within or on a host. The host is another organism.

The parasite uses the host’s resources to fuel its life cycle. It uses the host’s resources to maintain itself.

Parasites vary widely. Around 70 percent are not visible to the human eye, such as the malarial parasite, but some worm parasites can reach over 30 meters in length.

Parasites are not a disease, but they can spread diseases. Different parasites have different effects.


These live inside the host. They include heartworm, tapeworm, and flatworms. An intercellular parasite lives in the spaces within the host’s body, within the host’s cells. They include bacteria and viruses.

Endoparasites rely on a third organism, known as the vector, or carrier. The vector transmits the endoparasite to the host. The mosquito is a vector for many parasites, including the protozoan known as Plasmodium, which causes malaria.


These feed on other parasites in a relationship known as hyperparasitism. A flea lives on a dog, but the flea may have a protozoan in its digestive tract. The protozoan is the hyperparasite.

There are three main types of parasites.

Protozoa: Examples include the single-celled organism known as Plasmodium. A protozoa can only multiply, or divide, within the host.

Helminths: These are worm parasites. Schistosomiasis is caused by a helminth. Other examples include roundworm, pinworm, trichina spiralis, tapeworm, and fluke.

Ectoparasites: These live on, rather than in their hosts. They include lice and fleas.

There are many types of parasite, and symptoms can vary widely. Sometimes these may resemble the symptoms of other conditions, such as a hormone deficiency, pneumonia, or food poisoning.

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Some parasite-related problems, such as giardiasis and amebic dysentery, can cause abdominal pain.

Symptoms that might occur include:

  • skin bumps or rashes
  • weight loss, increased appetite, or both
  • abdominal pain, diarrhea, and vomiting
  • sleeping problems
  • anemia
  • aches and pains
  • allergies
  • weakness and general feeling unwell
  • fever

However, parasites can pass on a wide variety of conditions, so symptoms are hard to predict.

Often there are no symptoms, or symptoms appear long after infection, but the parasite can still be transmitted to another person, who may develop symptoms.

Many types of parasites can affect humans. Here are some examples of parasites and the diseases they can cause.


This tiny ameba can affect the eye, the skin, and the brain. It exists all over the world in water and soil. Individuals can become infected if they clean contact lenses with tap water.


This disease that comes from parasites that are spread by ticks. It affects the red blood cells. The risk is highest in summer in the Northeast and upper Midwest of the United States.


This is passed on by Balatidium coli, a single-cell parasite that usually infects pigs but can, in rare cases, cause intestinal infection in humans. It can be spread through direct contact with pigs or by drinking contaminated water, usually in tropical regions.


This affects the intestines. The blastocystis enters humans through the fecal-oral route. A person can get it by eating food or drink contaminated with human or animal feces where the parasite is present.


This affects the intestines. Coccidia is passed on through the fecal-oral route. It is found around the world. It can also affect dogs and cats, but these are different kinds. Dogs, cats, and humans cannot normally infect each other.


This is caused by the parasite Entamoeba histolytica. It affects the intestines. It is more likely in tropical regions and in areas with high population density and poor sanitation. It is transmitted through the fecal-oral route.


Giardia, or “beaver fever” affects the lumen of the small intestine. If humans ingest food or water contaminated with feces, dormant cysts may infect the body.

Isosporiasis or cystosporiasis

This disease is caused by the Cystoisospora belli, previously known as Isospora belli. It affects the epithelial cells of the small intestine. It exists worldwide and is both treatable and preventable. It is passed on through the fecal-oral route.


This is a disease that is passed on by parasites of the Leishmania family. It can affect the skin, the viscera, or the mucous membranes of the nose, mouth, and throat. It can be fatal. The parasite is transmitted by types of sandflies.

Primary amoebic meningoencephalitis (PAM)

This is passed on through a free-living ameba known as Naegleria fowleri. It affects the brain and the nervous system, and it is nearly always fatal within 1 to 18 days. It is transmitted through breathing in contaminated soil, swimming pools, and contaminated water, but not from drinking water.


Different types of plasmodium affect the red blood cells. It exists in tropical regions and is transmitted by the Anopheles mosquito.


This is caused by Rhinosporidium seeberi. It mainly affects the mucous of the nose, conjunctiva, and urethra. It is more common in India and Sri Lanka but can occur elsewhere. Polyps result in nasal masses that need to be removed through surgery. Bathing in common ponds can expose the nasal mucous to the parasite.


This is a parasitic pneumonia caused by the parasite Toxoplasma gondii. It affects the liver, heart, eyes and brain. It occurs worldwide. People can become infected after ingesting raw or undercooked pork, lamb, goat, or milk, or though contact with food or soil that is contaminated with cat feces.

A person with a healthy immune system will not usually have symptoms, but it can pose a risk during pregnancy and for those with a weakened immune system.


Also known as “trich” this is a sexually transmitted infection (STI) caused by the parasite Trichomonas vaginalis. It affects the female urogenital tract. It can exist in males, but usually without symptoms.

Trypanomiasis (Sleeping sickness)

This is passed on when the tetse fly transmits a parasite of the Trypanosoma family. It affects the central nervous system, blood, and lymph. It leads to changes in sleep behavior, among other symptoms, and it is considered fatal without treatment. It can cross the placenta and infect a fetus during pregnancy.

Chagas disease

This affects the blood, muscle, nerves, heart, esophagus and colon. It is transmitted through an insect bite. Over 300,000 people in the U.S. have the parasite that can lead to this disease.

Worms, or helminth organisms, can affect humans and animals.

Anisakiasis: This is caused by worms that can invade the intestines or the stomach wall. The worms are passed on through contaminated fresh or undercooked fish and squid.

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Roundworms can be passed on by raccoons.

Roundworm: Ascariasis, or a roundworm infection, does not usually cause symptoms, but the worm may be visible in feces. It enters the body through consuming contaminated food or drink.

Raccoon roundworm: Baylisascaris is passed on through raccoon stools. It can affect the brain, lungs, liver, and intestines. It occurs in North America. People are advised not to keep raccoons as pets for this reason.

Clonorchiasis: Also known as Chinese liver fluke disease, this affects the gall bladder. Humans can become infected after ingesting raw or poorly processed or preserved freshwater fish.

Dioctophyme renalis infection: The giant kidney worm can move through the wall of the stomach to the liver and eventually the kidney. Humans can become infected after eating the eggs of the parasite in raw or undercooked freshwater fish.

Diphyllobothriasis tapeworm: This affects the intestines and blood. Humans can become infected after eating raw fish that live wholly or partly in fresh water. Prevalence has increased in some parts of the developed world, possibly due to the growing popularity of sushi, salted fillets, ceviche, and other raw-fish dishes.

Guinea worm: This affects subcutaneous tissues and muscle and causes blisters and ulcers. The worm may be visible in the blister. As the worms are shed or removed, they enter the soil or water, and are passed on from there.

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Hookworms can cause intestinal disease.

Hookworm: These can cause intestinal disease. They lay their eggs in soil and the larvae can penetrate the skin of humans. Early symptoms include itching and a rash. They are most common in damp places with poor sanitation.

Hymenolepiasis: Humans can become infected by ingesting material contaminated by rodents, cockroaches, mealworms, and flour beetles.

Echinococcosis tapeworm: Cystic echinococcosis can lead to cysts in the liver and lungs, and alveolar echinococcosis can cause a tumor in the liver. Humans can be infected after eating foods contaminated by the feces of an infected animal, or from direct contact with an animal.

Enterobiasis pinworm: A pinworm, or threadworm, Enterobius vermicularis can live in the colon and rectum of humans. The worm lays eggs around the anus while a person sleeps, leading to itching. It spreads through the oral-fecal route.

Fasciolosis liver fluke: This affects the gall bladder and liver. It is common in countries where cattle or sheep are reared, but rare in the U.S. It can affect the liver and the bile ducts and it causes gastrointestinal symptoms. It passes from one mammal to another through snails. A person may get it from eating watercress, for example.

Fasciolopsiasis intestinal fluke: This affects the intestines. It can also transmitted when consuming contaminated water plants or water.

Gnathostomiasis: This causes swellings under the skin, and occasionally affects the liver, the eyes, and the nervous system. It is rare, but it can be fatal. It occurs in Southeast Asia. It is transmitted by eating freshwater fish, pigs, snails, frogs, and chicken.

Loa loa filariasis: Also known as loaisis, this is caused by the Loa loa worm, or African eye worm. It causes itchy swellings on the body. It occurs mainly in Central and West Africa and is transmitted through deerfly bites.

Mansonellosis: This is passed on through the bites of midges or blackflies. It affects the layers under the surface of the skin, but it can enter the blood. It can lead to angioedema, swellings, skin rash, fever, and joint problems. It is present in Africa and Cental America.

River blindness: Caused by a worm known as Onchocerca volvulus, this affects the eyes, skin, and other body tissues. It is found near fast flowing water. It is transmitted through the bite of a blackfly. It occurs in South America, but 90 percent of cases are in Africa.

Lung fluke: Also known as paragonimiasis, this affects the lungs, causing symptoms similar to those of tuberculosis (TB). However, it can reach the central nervous system, leading to meningitis. It is transmitted when eating undercooked or raw freshwater crabs, crayfishes, and other crustaceans. It is most common in parts of Asia.

Schistosomiasis, bilharzia, or snail fever: There are different types of schistosomiasis. They can affect the skin and internal organs. It results from exposure to fresh water that has snails in it that are infected with the blood fluke, or trematode worm. The worms are not found in the U.S. but they are common worldwide.

Sparganosis: Humans can become infected if they eat foods tainted with dog or cat feces that contains the larvae of a tapeworm of the Spirometra family. It can lead to a migrating abscess under the skin. It is rare.

Strongyloidiasis: This can lead to severe and possibly fatal immunodeficiency. The parasite penetrates through the skin and affects the lungs, skin, and intestines. It is passed on through direct contact with contaminated soil. It most occurs in tropical and subtropical regions.

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Different types of tapeworm can affect the intestines, the liver, or the lungs.

Beef and pork tapeworms: Taeniasis is caused by tapeworms of the taenia family. They affect the intestines. They are passed on by eating undercooked beef or pork.

Toxocariasis: A roundworm transmits this infection from animals to humans. It affects the eyes, brain, and liver. It is caused by accidentally swallowing the eggs of the parasite, for example, when young children play with soil. Nearly 14 percent of people in the U.S. have antibodies, suggesting that millions have been exposed. Most never have symptoms.

Trichinosis: This is caused by the roundworm of the Trichinella family. Infection can lead to intestinal symptoms, fever, and muscle aches. It is passed on by eating undercooked meat.

Whipworm: Also known as trichuriasis, whipworms live in the large intestine. Eggs are passed in feces. It is common all over the world. Humans can become infected when ingesting the eggs, for example on unwashed fruit or vegetables.

Elephantiasis lymphatic filariasis: This is transmitted through mosquito bites. The adult worms live in the lymph system. Infection can lead to lyphedema and elephantiasis, in which swelling can cause disfigurement and disability. In the Americas, it is passed on by the Culex quinquefasciatus mosquito.

Ringworm is sometimes mistaken for a worm, but it is not a worm. It is a fungal infection.

These are parasites that live on the outside of the body, such as fleas.

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Bed bugs are ectoparasites: They live on the outside of the body.

Bedbug: These can affect the skin and vision. They are found all over the world. Sharing clothing and bedding can spread infection. They may be present in newly rented accommodation and hotel rooms.

Body lice: These are common worldwide. Infection can spread through sexual activity, skin-to-skin contact, and sharing bedding or clothing.

Crab lice: These affect the pubic area and eyelashes. They are common all over the world and spread through sexual activity, skin-to-skin contact, and sharing bedding or clothing.

Demodex: These affect the eyebrow and eyelashes. They are common all over the world and can spread through prolonged skin contact.

Scabies: This affects the skin. It is common all over the world and can spread through sexual activity, skin-to-skin contact, and sharing bedding or clothing.

Screwworm: This is transmitted by a fly, and it affects skin and wounds. It is found in Central America and North Africa.

Head lice: These live on the scalp and affect the hair follicles. They are common all over the world and spread through head-to-head contact. A reaction to their saliva causes itching.

Parasites come in many shapes and sizes and can lead to a wide variety of symptoms and health issues. Some parasites are treatable and others are not.

To increase your chance of avoiding parasites:

  • find out which kind are prevalent in your area or in locations you may travel
  • take precautions, for example, using insect repellant in places where mosquitoes are common
  • be careful to eat only well-cooked fish and meat
  • when traveling, drink only water from bottles with a sealed top
  • take care when bathing in fresh-water lakes or rivers

If you have any symptoms, see a doctor.

According to the Centers for Disease Control and Prevention (CDC), the following parasitic infections are common in the U.S.:

  • neurocysticercosis
  • Chagas disease
  • toxocariasis
  • toxoplasmosis
  • trichomoniasis, or trich

The CDC is working to increase awareness of these diseases and to improve diagnostic testing.