The obligate intracellular protozoan Toxoplasma gondii is a coccidian whose primary hosts are felines. The organism has a broad range of secondary hosts, and is worldwide in distribution. T gondii exists in three forms: the tachyzoite, the tissue cyst and the oocyst. The tachyzoite, which is found mainly during the acute stage of infection, invades all types of mammalian cells. Toxoplasma multiplies at the site of invasion and thereafter spreads throughout the body via the bloodstream, eventually forming cysts in multiple tissues. The initial evidence that linked interferon-gamma, a major activator of macrophages, with Toxoplasma, came from studies indicating that infection with this parasite induced in vivo production of an antiviral substance which fulfilled many of the criteria used to define interferon. Cell-mediated immunity is thought to be a major factor in host resistance against Toxoplasma. As mentioned earlier, the activated macrophage is considered a primary effector cell of cellular immunity against this parasite.