A parasite is an organism that lives on or in another organism (usually referred to as the host), causing harm to the host. Parasites enter the body most often through the mouth or skin. Parasites that enter through the mouth are swallowed and can either remain in the intestine or burrow through the intestinal wall and invade other organs. Parasites that enter through the skin either bore directly through the skin or are introduced through the bites of infected insects. Some parasites enter through the skin or paws when an animal swims or bathes in water contaminated with the parasites.

Diagnosis of a parasitic infection usually requires samples of blood, feces, or urine for laboratory analysis. Repeated sample collections and examinations may be necessary to find the parasite. Less often, a tissue sample, or biopsy, is needed to find the parasite.

Some parasites, particularly those that are single-celled organisms, reproduce inside the host. Other parasites have complex life cycles, producing eggs or larvae that spend time in the environment or in an insect vector before becoming infective. If egg-laying parasites live in the digestive tract, their eggs may be found in the animal's feces when a sample is examined under a microscope.

Last full review/revision July 2011