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Dracunculiasis (Guinea worm). Source: CDC; Credit: Carter Center.

U.S. Department of Agriculture

The U.S. Department of Agriculture's Food Safety and Inspection Service defines a Parasite as an organism that derives nourishment and protection from other living organisms known as hosts.

Parasites may be transmitted from animals to humans, from humans to humans, or from humans to animals. Several parasites have emerged as significant causes of foodborne and waterborne disease.

These organisms live and reproduce within the tissues and organs of infected human and animal hosts, and are often excreted in feces.

Some common parasites are Giardia duodenalis, Cryptosporidium parvum, Cyclospora cayetanensis, Toxoplasma gondii, Trichinella spiralis, Taenia saginata (beef tapeworm), and Taenia solium (pork tapeworm).

Centers for Disease Control and Prevention

The Centers for Disease Control and Prevention's (CDC) Division of Parasitic Diseases (DPD) within CDC's National Center for Zoonotic, Vector-borne, and Enteric Diseases (ZVED) has assembled basic information about the nature of parasites.

About Parasites

A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. There are three main classes of parasites that can cause disease in humans:


Entamoeba histolytica, the parasite that causes amebiasis.
A microscope is necessary to view this parasite.
(Source: CDC)

Protozoa are microscopic, one-celled organisms that can be free-living or parasitic in nature. They are able to multiply in humans which contributes to their survival and also permits serious infections to develop from just a single organism. Transmission of protozoa that live in the human intestine to another human typically occurs by a fecal-oral route (for example, contaminated food or water, or person-to-person contact). Protozoa that live in the blood or tissue of humans are transmitted to humans by an arthropod vector (for example, through the bite of a mosquito or sand fly).

The protozoa that are infectious to humans can be classified into four groups based on their mode of movement:

  • Sarcodina – the ameba, e.g., Entamoeba;
  • Mastigophora – the flagellates, e.g., Giardia, Leishmania;
  • Ciliophora – the ciliates, e.g., Balantidium; and
  • Sporozoa – organisms whose adult stage is not motile e.g., Plasmodium, Cryptosporidium.


An adult Ascaris lumbriocoides worm.
Its size can range from15-35 cm.
Source: CDC

Helminths are large, multicellular organisms that are generally visible to the naked eye in their adult stages. Like protozoa, helminths can be either free-living or parasitic in nature. In their adult form, helminths cannot multiply in humans. There are three main groups of helminths (derived from the Greek word for worms) that are human parasites:

  • Flatworms (platyhelminths) – these include the trematodes (flukes) and cestodes (tapeworms).
  • Thorny-headed worms (acanthocephalins) – the adult forms of these worms reside in the gastrointestinal tract. The acanthocephala are thought to be intermediate between the cestodes and nematodes.
  • Roundworms (nematodes) – the adult forms of these worms can reside in the gastrointestinal tract, blood, lymphatic system or subcutaneous tissues. Alternatively, the immature (larval) states can cause disease through their infection of various body tissues.

Some consider the helminths to also include the segmented worms (annelids)—the only ones important medically are the leeches. Of note, these organisms are not typically considered parasites.

Dracunculiasis-LifeCycle CDC.gif.jpeg

Dracunculus medinensis Life Cycle

Humans become infected by drinking unfiltered water containing copepods (small crustaceans) which are infected with larvae of D. medinensis

Stomach 1.gif.jpeg . Following ingestion, the copepods die and release the larvae, which penetrate the host stomach and intestinal wall and enter the abdominal cavity and retroperitoneal space
Generic 2.gif.jpeg . After maturation into adults and copulation, the male worms die and the females (length: 70 to 120 cm) migrate in the subcutaneous tissues towards the skin surface
Generic 3.gif.jpeg . Approximately one year after infection, the female worm induces a blister on the skin, generally on the distal lower extremity, which ruptures. When this lesion comes into contact with water, a contact that the patient seeks to relieve the local discomfort, the female worm emerges and releases larvae
Generic 4.gif.jpeg . The larvae are ingested by a copepod
Generic 5.gif.jpeg and after two weeks (and two molts) have developed into infective larvae
Generic 6.gif.jpeg . Ingestion of the copepods closes the cycle
Stomach 1.gif.jpeg .

Geographic Distribution:
An ongoing eradication campaign has dramatically reduced the incidence of dracunculiasis, which is now restricted to rural, isolated areas in a narrow belt of African countries.


Adult louse; about as big as a sesame seed.
Source: CDC

Although the term ectoparasites can broadly include blood-sucking arthropods such as mosquitoes (because they are dependent on a blood meal from a human host for their survival), this term is generally used more narrowly to refer to organisms such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time (e.g., weeks to months). Arthropods are important in causing diseases in their own right, but are even more important as vectors, or transmitters, of many different pathogens that in turn cause tremendous morbidity and mortality from the diseases they cause.

Parasitic Infections

Parasitic infections cause a tremendous burden of disease in both the tropics and subtropics as well as in more temperate climates. Of all parasitic diseases, malaria causes the most deaths globally. Malaria kills approximately 1 million people each year, most of them young children in sub-Saharan Africa.

The neglected tropical diseases (NTDs), which have suffered from a lack of attention by the public health community, include parasitic diseases such as lymphatic filariasis, onchocerciasis, and Guinea worm. The NTDs kill approximately 500,000 people annually, largely in rural areas of low-income countries, but it is the enormous burden of morbidity due to these diseases that extracts the largest toll on endemic populations, with lost ability to attend school or work, retardation of growth in children, impairment of cognitive skills and development in young children, and the serious economic burden placed on entire countries.

However, parasitic infections also affect persons living in developed countries, including the United States.

  • Trichomonas is the most common parasitic infection in the U.S., accounting for an estimated 7.4 million cases per year.
  • Giardia and Cryptosporidium are estimated to cause 2 million and 300,000 infections annually in the U.S., respectively. Cryptosporidiosis is the most frequent cause of recreational water-related disease outbreaks in the U.S., causing multiple outbreaks each year.
  • There are an estimated 1.5 million new Toxoplasma infections and 400–4,000 cases of congenital toxoplasmosis in the U.S. each year; 1.26 million persons in this country have ocular involvement due to toxoplasmosis; and toxoplasmosis is the third leading cause of deaths due to food-borne illnesses (375+ deaths).

Related Parasitic Diseases

For more information on parasitic diseases see our A-Z Index of Parasitic Diseases.

Last modified: January 22, 2008
Last reviewed: February 27, 2007



(2011). Parasite. Retrieved from