Toxoplasma gondii is a species of parasitic protozoa in the genus Toxoplasma.

The definitive host of T. gondii is the cat, but the parasite can be carried by many warm-blooded animals (birds or mammals, including humans). Toxoplasmosis, the disease of which T. gondii is the causative agent, is usually minor and self-limiting but can have serious or even fatal effects on a fetus whose mother first contracts the disease during pregnancy or on an immunocompromised human or cat.

The life cycle of T. gondii has two phases. The sexual part of the life cycle (coccidia like) takes place only in cats, both domestic and wild (family Felidae), which makes cats the parasite's primary host. The second phase, the asexual part of the life cycle, can take place in other warm-blooded animals, including cats, mice, humans, and birds. The hosts in which asexual reproduction takes place is called the intermediate host. Rodents are the typical intermediate host.

In both kinds of hosts, the Toxoplasma parasite invades cells and forms a space called a vacuole. Inside this specialized vacuole, called a parasitophorous vacuole, the parasite forms bradyzoites, which are the slowly replicating versions of the parasite. The vacuoles containing the reproductive bradyzoites form cysts mainly in the tissues of the muscles and brain. Since the parasites are inside cells, they are safe from the host's immune system, which does not respond to the cysts.^{[citation needed]}