Parasites are organisms that live in or on the body of a host organism and are dependent on the host for completion of their life cycle, to the detriment of the host.


Parasitic roundworm (Ascaris lumbricoides) in a gloved hand. A. lumbricoides is a parasite of the human intestine.

Parasitic roundworm (Ascaris lumbricoides) in a gloved hand. A. lumbricoides is a parasite of the human intestine. Female roundworms can lay up to 200,000 eggs per day, which are excreted in the feces and ingested by a new host through contaminated water or food.
(Clouds Hill Imaging Ltd./Science Source)

Because parasites can live inside the human body for years without making their presence known, they are more common than one might think. According to one study, approximately half of all Americans carry at least one parasite that may or may not cause symptoms, depending on the type of parasite and the individual's health status.

The U.S. Centers for Disease Control and Prevention (CDC) requires that five parasitic diseases be reported: Chagas disease, cystericercosis, toxocariasis, toxoplasmosis, and trichomoniasis. Trichomoniasis is the most common parasite in the United States with about 7.4 million cases reported annually.


Parasites may be plants, animals, viruses, bacteria, or fungi. They feed either on their host directly or upon its surplus fluids. They harm their hosts because they consume food, damage tissues and cells, and eliminate toxic wastes that can make the host sick. Some parasites, known as endoparasites, live inside their host, whereas ectoparasites live on the outside of their host. Organisms in which parasites reach maturity are called definitive hosts, and hosts harboring parasite stages are called intermediate hosts. Organisms that spread parasite stages between hosts are called vectors. Parasites can spread through the ingestion of contaminated food or water, from contact with infected animals, including pets (zoonotic parasites), through the bite of an infected insect, through transmission of infected blood, by inhaling dust containing eggs of parasites, or by direct contact through a cut or the skin.

Full-time, or obligatory, parasites have an absolute dependence on their hosts. Examples of this type are viruses, which can only live and multiply inside living cells, and tapeworms, which can only live and multiply inside other species. Part-time, or facultative, parasites, such as wood ticks, have parasitic and free-living stages in their life cycle and are only temporary residents of their hosts.

Parasites can generally be divided into large or macroparasites and small or microparasites. Large parasites can be seen with the naked eye. They include internal parasites such as roundworms, flukes, and tapeworms and external parasites such as ticks, lice, and fleas. Some large internal parasites lay eggs on the intestinal walls. As the eggs hatch, the young larvae feed on the food in the intestinal tract. Then they grow, reproduce, and start the cycle all over again. They sometimes dig through the digestive tract to get into the bloodstream, muscles, and other organs. These types of parasites can cause malnutrition and anemia because they tend to rob the body of essential nutrients; however, macroparasites that employ one or more intermediate hosts, such as flukes, tapeworms, or roundworms, have highly effective transmission stages but usually have only a limited effect on the population of the host.

Microparasites include viruses, bacteria, protozoa can only be seen with a microscope. Microparasites can migrate virtually anywhere in the body (into the bloodstream, muscles, and even vital organs such as the brain, the lungs, or the liver) and do substantial damage. Microparasites that are transmitted directly between infected hosts, such as rabies or distemper, target herd animals with a high host density and can significantly reduce population levels.

Risk factors

Risk factors that increase the chance for becoming infected with a parasite:

Causes and symptoms

People can acquire a parasitic infection in many ways, including:


There are more than 100 types of human parasites. The following describe some of the most common species in the United States.

ARTHROPODS. Common arthropods such as ticks, mites, fleas, and lice may cause intense itching in affected areas. Ticks are particularly able to transmit serious disease such as Lyme disease and Rocky Mountains potted fever. Other parasites, spread by mosquitoes, cause more serious diseases such as western and eastern equine encephalitis, malaria, Dengue fever, and yellow fever.

INTESTINAL PARASITES. Some of the most common intestinal parasites include:

CENTRAL NERVOUS SYSTEM PARASITIC INFECTIONS. Toxoplasma gondii is the most common parasite that invades the central nervous system (CNS). Humans become infected with this organism by eating raw or undercooked meat or by handling infected cat litter, which can contain eggs. Pregnant women who are infected may miscarry or deliver stillborn babies. Infected babies are born with congenital toxoplasmosis and have symptoms that include eye inflammation, blindness, jaundice, seizures, abnormally small or large heads, and mental retardation. In people with weakened immune systems, such as people with AIDS, toxoplasmosis can affect the whole body, causing inflammation, convulsions, trembling, headache, confusion, paralysis in half of the body, or coma.


The effects of parasites on their hosts depend on the health of the host, as well as the severity of the infestation. In diseased, old, or poorly fed individuals, parasite infestations can be fatal, but parasites do not typically kill their hosts, although they can slow growth and cause weight loss. Some plant parasites kill their hosts and then live on its decomposing remains, and certain species of hymenopteran insects are parasitoids, whose larva feeds within the living body of the host, eventually killing it.

Parasitic infections are difficult to diagnose because many individuals exhibit only vague symptoms or no symptoms at all. The following symptoms, although not unique to parasitic infections, may suggest that the individual is infected:

Other symptoms of parasitic infections include anemia, blood in the stool, bloating, diarrhea, gas, loss of appetite, intestinal obstruction, nausea, vomiting, sore mouth and gums, excessive nose picking, grinding teeth at night, chronic fatigue, headaches, muscle aches and pains, shortness of breath, skin rashes, depression, and memory loss.


The following tests may be used to help doctors diagnose parasitic infections:

Infected patients who are treated with anti-parasitic drugs or herbal remedies ought to be retested twice at the end of the treatment program; the tests should be given one month apart.


Insect infestations

Infestations with lice, ticks, or fleas can be controlled by insecticides and attention to hygiene and household or environmental contact.

A type of white blood cell that increases in number in response to certain medical conditions, such as allergy or parasitic infection.
Also called the bowels and divided into the large and small intestine. They extend from the stomach to the anus, where waste products exit the body. The small intestine is about 20 ft (6.1 m) long and the large intestine, about 5 ft (1.5 m) long.
Single-celled microorganisms belonging to the subkingdom Protozoa that are more complex than bacteria. About 30 protozoa cause diseases in humans.
Any disease or parasite of animals that can be transmitted to humans under natural conditions. Lyme disease and rabies are examples.
Intestinal parasites

Treatment for intestinal parasites usually involves anti-parasitic drugs. Depending on the severity of the condition and the species involved, treatment may include one (or more) of the following drugs: albendazole, furazolidone, iodoquinol, mebendazole, metronidazole, niclosamide, nitrazoxinide, paromomycin, pyrantel pamoate, pyrimethamine, quinacrine, sulfadiazine, or thiabendazole.

To prevent re-infection and transmission of disease, thorough cleaning of hands, clothes, sheets, and toys is recommended. Treatments should involve all members of the family and repeated treatments may be necessary.

CNS parasitic infections

Babies or HIV-infected individuals with toxoplasmosis are often given spiramycin or sulfadiazine plus pyrimethamine. Treatment may be continued indefinitely for HIV/AIDS patients to prevent recurrence.

Alternative treatment

Nutritional therapy

The following dietary changes may help prevent or treat parasitic infections:

Herbal therapy

Herbal treatment should be given in combination with supportive dietary treatment and continued until the worms are completely eradicated. The following herbs are helpful in treating parasitic infestations:

Ayurvedic medicine

Momordica charantia (bitter melon) is a safe remedy for pinworm infection. The melon is a vegetable shaped like a cucumber with a bitter taste. It can be found in most Oriental markets. It should be sliced thinly and eaten raw with other vegetables to reduce its bitter taste. Daily consumption of one to two bitter melons for seven to 10 days can eliminate pinworm infection. Patients may want to repeat the regimen after several months to prevent re-infection. Chinese herbal combinations also help treat parasitic infections by supporting the gastrointestinal system, stimulating immune response, and killing parasites.



Though parasitic infections are difficult to diagnose, complete recovery from infestation can be achieved with appropriate therapy. Because re-infestation is common, multiple treatments may be necessary.


The following measures can help prevent parasitic infections:

See also Dengue fever ; HIV/AIDS ; Lyme disease ; Malaria ; Sanitation ; Viruses ; Yellow fever .



Drisdelle, Rosemary. Parasites: Tales of Humanity's Most Unwelcome Guests. Berkeley: University of California Press, 2010.

Dunn, Rob R. The Wild Life of Our Bodies: Predators, Parasites, and Partners That Shape Who We Are Today. New York: Harper, 2011.

McGuire, Robert A. Parasites, Pathogens, and Progress: Diseases and Economic Development. Cambridge, MA: MIT Press, 2011.

Zuk, Marlene. Riddled with Life: Friendly Worms, Ladybug Sex, and the Parasites That Make Us Who We Are. New York: Mariner Books, 2008.


MedlinePlus “Parasitic Disease.” medlineplus/parasiticdiseases.html (accessed October 29, 2012).

Centers for Disease Control and Prevention. “Parasites.” (accessed October 29, 2012).

Merck Manual Staff. “Approach to Parasitic Infections.” (accessed October 29, 2012).

World Health Organization. “Initiative for Vaccine Research: Parasitic Diseases.” (accessed October 29, 2012).


Centers for Disease Control and Prevention (CDC), 1600 Clifton Rd., Atlanta, GA, 30333, (404) 639-3534, (800) CDC-INFO (800-232-4636); TTY: (888) 232-6348,, .

National Institute of Allergy and Infectious Diseases Office of Communications and Government Relations, 6610 Rockledge Dr., MSC 6612, Bethesda, MD, 20892-6612, (301) 496-5717, (866) 284-4107 or TDD: (800)877-8339, Fax: (301) 402-3573,, .

World Health Organization, Avenue Appia 20, 1211 Geneva 27, Switzerland, 2241 791 21 11, Fax: 2241 791 31 11,, .

Revised by Rhonda Cloos, RN
Revised by Tish Davidson, AM

  This information is not a tool for self-diagnosis or a substitute for professional care.