Toxoplasma gondii is an obligate intracellular parasite capable of infecting humans and a variety of other warm-blooded animals. The organism is distributed widely in nature and may infect as many as 1 of 3 persons worldwide. Most Toxoplasma infections are either asymptomatic or otherwise benign; notable exceptions include the infections of the developing fetus whose mother acquires an acute T gondii infection during gestation and of immunosuppressed patients, such as those with the acquired immunodeficiency syndrome (AIDS), in whom the illness may be severe or even fatal. The cat and other felines (such as the lynx) appear to be the only definitive hosts for this protozoan.