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Toxoplasma gondii is a parasite of birds and mammals. Cats are the only definitive host and thus the only source of infective oocysts, but other mammals and birds can develop tissue cysts. Although feline infections are typically asymptomatic, infection during human pregnancy can cause severe disease in the fetus. Cat owners can reduce their pets' exposure risk by keeping all cats indoors and not feeding them raw meat. Humans usually become infected through ingestion of oocyst-contaminated soil and water, tissue cysts in undercooked meat, or congenitally. Because of their fastidious nature, the passing of non-infective oocysts, and the short duration of oocyst shedding, direct contact with cats is not thought to be a primary risk for human infection.