Toxoplasmosis is caused by the protozoan Toxoplasma gondii, a sporozoan. Toxoplasmosis is a disease transmitted by cats, which are the definitive hosts of the parasite. Cats eliminate toxoplasma oocysts in feces; the oocysts are extremely resistant and remain viable for months in the environment. Human beings are infected when ingesting water or food contaminated by oocysts or when making contact with contaminated objects. Humans can also become infected by eating meat of animals like pork, cow and sheep, which can be intermediate hosts too. Vertical transmission, from mother to offspring, may also occur.
In toxoplasmosis the cystic form of the parasite invades tissues of the body, including the brain and the retina. The infestation is potentially fatal. In congenital toxoplasmosis the child may present blindness and mental retardation. The disease is especially severe when occurring as an opportunistic disease in AIDS patients.