Composite image of illustrations of the parasitic protozoan Toxoplasma gondii, the causative agent of toxoplasmosis, in tachyzoite stage, and a photograph of a street cat, the Toxoplasma parasite's definitive host. Toxoplasmosis is usually a minor and self-limiting disease. It can, however, have serious or even fatal effects on an unborn foetus if the mother contracts toxoplasmosis during pregnancy.