A person with toxoplasmosis has an infection with a parasite, called Toxoplasma gondii. The infection only causes illness in those with weak immune systems.

Toxoplasmosis rarely causes symptoms in those with normal immune systems. About 70 percent of adults in the US have had toxoplasmosis, but did not develop symptoms during the infection.

Congenital toxoplasmosis is an infection that occurs in a fetus. The parasite can enter the fetus if the mother is infected during pregnancy.