Parasitic Diseases: Overview

Parasitic diseases are caused by infestation by such organisms as protozoa (one-celled animals), worms, or insects. These diseases are widespread in Africa, Southeast Asia, and Central and South America, especially among children.

What Are Parasitic Diseases?

The Giardia lamblia protozoan that causes giardiasis.

The Giardia lamblia protozoan that causes giardiasis.
Illustration by Frank Forney. © 2016 Cengage Learning®.

As of 2015, most of the world's nearly 7 billion people were infected with parasites, which are primitive animals that live in or on the bodies of humans, other animals, or insects. Often the parasites do little damage, and people may be unaware that they have them. In any given year, however, more than 1 billion people, many of them children, fall sick with parasitic diseases, and millions of them die.

Where Do Parasitic Illnesses Occur?

Parasites live everywhere, but they thrive in warm, moist environments. Parasitic diseases are most common in sub-Saharan Africa, the Indian subcontinent, Southeast Asia, and Central and South America. Some regions and some nations in these parts of the world are too poor to take measures that could prevent many parasitic infections—such as building water and sewage treatment plants, controlling mosquito populations, and providing adequate medical care. Furthermore, in many of these places, parasitic diseases make so many people weak, ill, and unable to work that they slow economic development and help keep the regions impoverished.

Travelers or military personnel from industrialized nations may pick up parasitic infections when they visit tropical areas. In addition, parasites can infect people who live in industrialized nations if they eat food that was produced in tropical areas. Examples of such parasites are Cyclospora and Cryptosporidium, both protozoans.

What Are the Most Common Parasitic Diseases?

The most common parasitic diseases worldwide are ascariasis (AScuh-RYE-ah-sis). The intestinal roundworm Ascaris lumbricoides causes ascariasis, and is estimated to infect more than 1 billion people, although it often does little damage. Malaria (a disease caused by Plasmodium a protozoan, transmitted to humans by mosquitoes) has a much greater impact. According to the Centers for Disease Control (CDC), the malaria-causing agent is responsible for 500 million illnesses per year worldwide and more than 1 million deaths. About half of those deaths occur in children under five years of age. Schisto-soma blood flukes cause schistosomiasis (SHIS-to-so-MY-a-sis), which results in 120 million cases of illness, 20 million of them severe, according to the CDC.

Other parasitic diseases that are estimated to afflict 1 million or more persons per year are filariasis, amebiasis, Chagas disease, leishmaniasis, and African sleeping sickness (trypanosomiasis).


Schistosomiasis, also known as bilharziasis (bil-har-ZYE-uh-sis) or “snail fever,” is a leading cause of illness in tropical regions. According to the World Health Organization, 61.6 million people worldwide were reported to have been treated for schistosomiasis in 2014. In terms of impact, this disease is second only to malaria as the most devastating parasitic disease, and is considered one of the Neglected Tropical Diseases (NTDs) * .

Schistosomiasis is caused by parasitic worms that live in certain types of freshwater snails. The worm emerges from the snail and contaminates freshwater canals, rivers, streams, ponds, or lakes. The disease is most common in tropical regions: Africa, South America, the Caribbean (risk in the Caribbean is low), the Middle East, southern China, and parts of Southeast Asia and the Philippines and Laos. The worms that cause schistosomiasis are not found in the United States.

Travelers visiting countries where schistosomiasis occurs should avoid wading, swimming, or bathing in any body of freshwater. The disease is not acquired by contact with saltwater (oceans or seas) or swimming in chlorinated pools. Schistosomiasis is not transmitted by swallowing contaminated water, however, contact with the water on the mouth or lips could cause infection.


Not all parasites live inside the human body; some live on the skin or outgrowths of the skin. They are called ectoparasites, from the Greek Ecto which means “outer.” Ectoparasites include fleas, lice, and the mites that cause scabies (SKAY-beez).

How Do Parasitic Diseases Spread?

In many cases, people get parasitic infections as a result of bathing in, swimming in, or drinking water that contains parasites; by eating food that has not been cooked thoroughly; or by coming into contact with untreated sewage, which can happen when human waste is used to fertilize fields. Parasitic diseases can also spread when people who handle food do not wash their hands thoroughly after using the bathroom.

Many impoverished nations are undergoing rapid urbanization, meaning that many people are crowded together in fast-growing cities that may lack sewage treatment facilities. Raw (untreated) sewage may be dumped into rivers whose water is also used for drinking, bathing, washing, and cooking. Parasitic diseases spread easily in such conditions.

In addition to poor sanitation, insects and animals spread some parasitic diseases. Mosquitoes, for instance, spread the protozoan that causes malaria. Tsetse flies spread the protozoan that causes African trypanosomiasis (TRI-pan-o-so-MY-a-sis), also called African sleeping sickness. Domestic animals spread beef and pork tapeworms.

How Do People Know They Have Parasitic Diseases?


The symptoms of parasitic diseases vary widely depending on the parasite. Parasitic infections may cause symptoms including (but not limited to) the following:

How Do Doctors Diagnose and Treat Parasitic Diseases?


Parasitic diseases can be difficult to diagnose because there is often a time lag between the parasite's entrance into the body and the appearance of the first symptoms; also, evidence of the presence of many parasites does not always appear in the routine blood tests that doctors perform. In addition, people with parasites are prone to getting bacterial infections as well, which may give doctors the impression that the bacteria alone are the cause of the patient's illness.

Certain blood tests, however, can help with diagnosis, as can asking the patient about any recent visits to areas where parasitic infections are common. In addition, medical professionals can sometimes see parasites if they examine samples of the patient's stool or blood under a microscope.


There are a variety of medications that can kill most parasites, although not all parasites can be treated with medication, and some, like the malaria parasite, have developed resistance to medications. Because different parasites respond best to different medications, individuals should consult a medical professional to assure proper treatment.

Can Parasitic Diseases Be Prevented?

It is possible to prevent many parasitic diseases, but unfortunately there were no vaccines that can prevent any of these diseases as of 2016. Public authorities that build sewage and water treatment systems play a major part in preventing these diseases. Disease prevention is also achieved by controlling insect populations that spread some parasitic diseases and by teaching people always to wash their hands thoroughly after using the bathroom and before handling food. Cooking meat to safe temperatures will prevent trichinosis, and good personal hygiene will prevent most ectoparasite infections. To control many parasitic infections effectively, a combination of medications and sanitary improvements are necessary.

See also Bites and Stings • Brain Parasites • Chagas Disease • Cyclosporiasis and Cryptosporidiosis • Elephantiasis • Fever • Filariasis • Giardiasis • Hookworm • Intestinal Parasites • Malaria • Roundworm Infection • Skin Parasites: Overview • Tapeworm • Toxoplasmosis • Trichinosis


Books and Articles

Despommier, Dickson D. People, Parasites, and Plowshares: Learning from Our Body's Most Terrifying Invaders. New York: Columbia University Press, 2013.

Packard, Randall M. The Making of a Tropical Disease: A Short History of Malaria. Baltimore, MD: John Hopkins University Press, 2011.

Singh, Sunit K., editor. Human Emerging and Re-emerging Infections. Hoboken, NJ: John Wiley and Sons, 2015.


Centers for Disease Control and Prevention. “Parasites.” (accessed April 8, 2016).

MedlinePlus. “Parasitic Diseases.” U.S. National Library of Medicine, National Institutes of Health. (accessed April 8, 2016).

National Institute of Allergy and Infectious Diseases. “Parasitic Infections.” (accessed April 8, 2016).


American Society of Tropical Medicine and Hygiene. One Parkview Plaza, Suite 800, Oakbrook Terrace, IL 60181. Telephone: 847-686-2238. Website: (accessed April 10, 2016).

Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: (accessed April 8, 2016).

U.S. Food and Drug Administration. 10903 New Hampshire Ave., Silver Spring, MD 20993. Toll-free: 888-463-6332. Website: (accessed April 8, 2016).

World Health Organization. Avenue Appia 20, 1211 Geneva 27, Switzerland. Telephone: 41-22-791-21-11. Website: (accessed April 8, 2016).

* Neglected Tropical Diseases (NTDs) are a group of parasitic and bacterial diseases that cause substantial illness globally. They affect the world's poorest populations and impair physical and cognitive development, contribute to illness and death, make it difficult to farm or earn a living, and limit productivity in the workplace.

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

(MLA 8th Edition)