Parasite Primer

Symptoms and Sources of Infection

Millions of Americans are affected by parasites. From protozoa to roundworms, tapeworms to flukes, there are four main classifications of parasites that infect humans. Symptoms may be mild or severe,

Learn about the various types of parasites, symptoms, and the sources of infection and spread.

Ancylostoma hookworm, illustration

Kateryna Kon / Science Photo Library / Getty Images


Protozoa, microscopic single-celled organisms, are the most common type of parasite in the United States. Unlike other kinds of parasites, they reproduce very rapidly. They do this in the intestines, and can travel to other organs such as the liver, lungs, pancreas, and heart. Protozoa have an indefinite life span.

  • Giardia (Giardia duodenalis) originates in infected humans and animals. It is transmitted through water, food, and contact with feces, often due to poor hygiene and handwashing. Giardia infects the small intestine. It can take up to three weeks before symptoms of giardia appear. Giardia can result in asymptomatic intestinal infection. It can also produce watery diarrhea, foul-smelling stools, nausea, stomach cramps, bloating, gas, fatigue, and weight loss. Multiple stool samples are often necessary for diagnosis.
  • Cryptosporidium parvum is often transmitted by contact with human feces that contain infectious cysts, for example through poor handwashing. Cryptosporidium parvum is transmitted in water. It can be asymptomatic in people with healthy immune systems, or symptoms can include watery diarrhea, nausea, cramps, and fever. Symptoms usually last two to three weeks. In people who are immunocompromised, there can be severe diarrhea with weakness and weight loss, which can be life-threatening.
  • Cyclospora cayetanensis is mainly seen in travelers. Outbreaks are usually associated with food or water contaminated with feces. Symptoms are similar to giardiasis. Symptoms come and go and can include diarrhea, frequent watery stools, weight loss, fatigue, bloating, and vomiting. It can also affect the gallbladder, especially in people with compromised immune systems.
  • Entamoeba histolytica is spread through contaminated water or food. Insects, such as flies and cockroaches, can carry the cysts. It may take days or weeks after infection before symptoms appear. It can spread through the digestive tract and travel to other organs. Most often, the infected person is a symptomless carrier. It can cause stomach pain, cramps, and diarrhea. If there is tissue destruction in the large intestine, there can be low-grade fever with bloody diarrhea.
  • Toxoplasma gondii is an infection that is often associated with cats, though it can be transmitted via contaminated water or soil, or via undercooked meat. Symptoms include flu-like symptoms such as fever, swollen lymph nodes, and fatigue.
  • Trichomonas vaginalis is transmitted through sexual contact. There are often no symptoms. It can cause genital discharge, genital itching or pain, and painful urination. In women, it can cause changes in vaginal discharge, including a fishy smell.

Roundworms and Hookworms

These are unsegmented worms. They produce eggs that require incubation in soil or in another host before parasitizing humans.

  • Roundworm (Ascaris lumbricoides) is symptomless in many people. It is estimated that about a billion people in the world may be infected with roundworms. The source of infection is fecal contamination of soil and food. Adult roundworms live in the small intestine and can exit through the nose of the infected person. Occasionally, there is obstruction of the pancreatic or bile duct, appendix, or small intestine. Dry cough and abdominal discomfort may occur. Diagnosis is by stool exam for eggs and a blood test.
  • Hookworm (Necator americanus) is transmitted through unbroken skin by walking barefoot. Hookworms travel into the blood and through the lung and intestines. Hookworm infection is usually symptomless. There may be itching at the area of skin penetration. There can be digestive symptoms. The worms attach to and suck blood from the small intestine, which can lead to iron deficiency anemia in severe infections.
  • Pinworm (Enterobius vermicularis) infection is common in the United States. It is transmitted through contaminated food and water. The worms live in the intestines near the rectum and travel at night outside to the skin around the anus. From there it can be transmitted through person-to-person contact. It can be symptomless. There is often itching at night around the anus. Other symptoms may include vaginitis, abdominal pain, bed-wetting, loss of appetite, and irritability. To help make a diagnosis, tape or a swab may be used to collect any worms that may be in the area around the anus. Adult worms may be seen with the unaided eye but examination with a microscope may be necessary.
  • Whipworm (Trichuris trichiura) is a large intestine parasite that rarely shows symptoms. It is transmitted by ingestion of the eggs in soil or on vegetables. Symptoms of heavy infection include diarrhea, stomach pain, rectal prolapse, and stunted growth.
  • Trichinella (Trichinella spiralis) infection—commonly called trichinosis—is often due to eating undercooked meat, especially pork. When the roundworms are in the intestines, symptoms may include intestinal pain, vomiting, and diarrhea. When they travel from the intestines into the muscle, symptoms may include severe muscle pain, facial swelling, weakness, and fatigue. In severe cases, trichinosis can affect the nervous system, respiratory system, and heart.


Tapeworms are flat, segmented, and shaped like ribbons. Tapeworms are the largest intestinal parasites and can grow to up to several feet or more in length. Tapeworm larvae can be found in undercooked meat or fish. From larvae, worms develop in the body and attach to the small intestine. They survive there by absorbing nutrients from ingested foods. Tapeworm infection is often symptomless, and treatment involves medication that targets the worm.

  • Pork tapeworm (Taenia solium) infection most commonly occurs after eating undercooked pork, smoked ham, or sausages containing larvae. Adult worms attach to the intestines. Symptoms are similar to infection with beef tapeworm (see below). Larvae can travel to subcutaneous tissue, muscle, the central nervous system, and/or the eye, where they eventually form cysts which can lead to blindness, seizures, neurological deficits, and hydrocephalus (swelling of the head).
  • Beef tapeworm (Taenia saginata) infection occurs most commonly after eating undercooked beef containing the larvae. It can live in the intestines for years and grow to a length of more than 30 feet. It is usually symptomless, although occasionally presents as abdominal discomfort, loss of appetite, weight loss, and diarrhea. Segments can crawl out of the anus.
  • Fish tapeworm (Dibothriocephalus latus) infection is most commonly due to eating freshwater fish containing larvae. Fish tapeworms can grow more than 30 feet in length. Symptoms are nonspecific abdominal symptoms, such as loss of appetite, diarrhea, nausea, and vomiting. Vitamin B12 deficiency can also occur, leading to pernicious anemia.
  • Clonorchis sinensis is another worm transmitted by eating raw fish. The worms live in the gallbladder area, so complications can include bile duct stones, gallbladder stones, and other gallbladder disease.

Flukes or Flatworms

Flukes or flatworms are leaf-shaped worms that attach to the host using abdominal suckers. They usually begin their life cycle as snails, and then as larvae they infect fish, vegetation, or humans. Flatworms can travel to the lungs, intestines, heart, brain, and liver. Eggs can cause inflammation by releasing toxins that damage tissues.

  • Intestinal fluke (Fasciolopsis buski) worms live in the small intestine. They can cause intestinal ulcers and allergic reactions. Common symptoms are vomiting, diarrhea, nausea, and stomach pain. Intestinal fluke contamination comes from eating infected water vegetables, such as water chestnuts, bamboo shoots, and watercress.
  • Oriental lung fluke (Paragonimus westermani) is found predominantly in Asian countries. These worms can penetrate the intestines and travel to the brain or lungs. Symptoms of infection include irrepressible coughing fits and bloody sputum. Sources of these worms include undercooked crabs and crayfish.
  • Sheep liver fluke (Fasciola hepatica) is most commonly transmitted from fresh watercress. The worm attaches to the gallbladder and bile ducts, causing inflammation and local trauma. Symptoms include jaundice, fever, coughing, vomiting, and abdominal pain.
  • Blood flukes (Schistosoma spp.) are transmitted by swimming or bathing in contaminated water. They burrow into the skin and migrate to the heart, lungs, liver, or bladder. They can live in the body for years.

Also read about pargonimusschistosomiasis, and cryptosporidium

19 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Cathy Wong
Cathy Wong is a nutritionist and wellness expert. Her work is regularly featured in media such as First For Women, Woman's World, and Natural Health.