A parasitic disease is an infectious disease caused or transmitted by a parasite. Many parasites do not cause disease per se. Parasitic diseases can affect practically all living organisms, from plants to man. The study of parasitic diseases is called by parasitology.

Some parasites like Toxoplasma gondii can cause Toxoplasmosis as a parasitic disease directly caused by the protozoan. In contrast, some of the symptoms caused by parasites are due to the toxins produced by the parasites, rather than the parasites themselves.

See also

• Human parasitic diseases for medical parasitology