Toxoplasma gondii is a ubiquitous protozoan parasite that is extremely widespread and of great medical importance, infecting all mammalian cells and responsible for human and veterinary dis- eases. It was initially described in Tunis by Nicolle and Manceaux (1908) in the tissues of the gundi (Ctenodoactylus gundi) and later in Brazil by the microbiologist Alfonso Splendore (1908) in the rab- bit. Its identification was rapidly followed by the recognition that it was a human pathogen. In this regard, the Italian bacteriologist Castellani (1914) was probably the first to describe a T. gondii-like parasite in smears of blood and spleen from a 14-year-old Singha- lese boy who died from a disease characterized by severe anemia, fever, and splenomegaly. However, it was not until the 1960s and 1970s that the parasite was identified as a coccidian and the cat rec- ognized as the definite host. Toxoplasma belongs to the phylum Apicomplexa, which con- tains many other protozoan pathogens of human and veterinary importance, such as Plasmodium spp. (malaria), Cryptosporidium spp. (cryptosporidiosis), and Eimeria spp. (poultry coccidiosis). Disease can occur through acute infection after recent contact with T. gondii cysts or oocysts or through endogenous reactiva- tion. Primary infection is usually subclinical, but in some patients cervical or occipital lymphadenopathy or ocular disease is present. Infection acquired during pregnancy can cause severe damage to the fetus if Toxoplasma crosses the placental barrier, and it causes abortion or congenital birth defects if the mother becomes infected for the first time shortly before or during pregnancy. In AIDS patients and others who are immunocompromised persons, reactivation of latent disease can cause life-threatening encephalitis. Ocular infection by Toxoplasma is a major cause of retinochoroiditis in several geographic areas in both immunocom- petent and immunocompromised persons.