Toxoplasma gondii (T. gondii), one of the most common parasites in developed countries, is an obligate intracellular parasitic one-celled eukaryote. It can cause infectious disease toxoplasmosis in all warm-blooded animals, but felids such as domestic cats are the only known definitive hosts where the parasite may undergo sexual reproduction. Infection may cause a serious and occasionally fatal illness in people with weakened immunity. Research on human vaccines is ongoing.