Toxoplasma gondii (tŏk'sə-plāz'mə gŏn'dē-ī') is an obligate, intracellular, parasitic protozoan that causes the disease toxoplasmosis.
Found worldwide, T. gondii is capable of infecting virtually all warm-blooded animals. In humans, it is one of the most common parasites; serological studies estimate that up to a third of the global population has been exposed to and may be chronically infected with T. gondii, although infection rates differ significantly from country to country. Although mild, flu-like symptoms occasionally occur during the first few weeks following exposure, infection with T. gondii generally produces no symptoms in healthy human adults. However, in infants, HIV/AIDS patients, and others with weakened immunity, infection can cause serious and occasionally fatal illness (toxoplasmosis).
Although T. gondii can infect, be transmitted by, and asexually reproduce within humans and virtually all other warm-blooded animals, the parasite can sexually reproduce only within the intestines of members of the cat family (felids). Felids are therefore defined as the definitive hosts of T. gondii, with all other hosts defined as intermediate hosts.
T. gondii has been shown to alter the behavior of infected rodents in ways thought to increase the rodents' chances of being preyed upon by cats. Because cats are the only hosts within which T. gondii can sexually reproduce to complete and begin its lifecycle, such behavioral manipulations are thought to be evolutionary adaptations to increase the parasite's reproductive success, in one of the manifestations the evolutionary biologist Richard Dawkins attributes to the "extended phenotype". Although numerous hypotheses exist and are being investigated, the mechanism of T. gondii–induced behavioral changes in rodents remains unknown.