Toxoplasmosis is an infectious disease caused by the protozoan Toxoplasma gondii. The infection results from a parasitic association with a human host.

Cats are the primary carrier of the protozoan Toxoplasma gondii. In the United States, approximately 30% of cats are at some time infected by Toxoplasma gondii. Cattle, sheep, or other livestock can also excrete a form of the protozoan known as an oocyst. Although oocysts are not capable of producing an infection, they are important because they act to preserve the infectious capability of the protozoan during exposure to inhospitable environments. In this capacity they are analogous to the bacterial spore. Oocysts are often capable of resuscitation into the infectious form after prolonged periods of exposure to adverse environments.

Humans can also become infected by eating fruits and vegetables that have themselves become contaminated when irrigated with untreated water contaminated with oocyte-containing feces.

Humans typically contract toxoplasmosis by eating cyst-contaminated raw or undercooked meat, vegetables, or milk products. The protozoan can also be spread from litter boxes or a sandbox soiled with cat feces. In all cases, the agent that is ingested can be the inactive oocyst or the actively growing and infectious egg form of the parasite.

In the human host, the parasite is able to grow and divide. This causes the symptoms of the infection.

Symptoms of toxoplasmosis include a sporadic and reoccurring fever, muscle pain, and a general feeling of malaise. Upon recovery, a life-long immunity is conferred. In some people, the disease can become chronic and cause an inflammation of the eyes, called retinochoroiditis, that can lead to blindness, severe yellowing of the skin and whites of the eyes (jaundice), easy bruising, and convulsions. As well inflammation of the brain (encephalitis), one-sided weakness or numbness, mood and personality changes, vision disturbances, muscle spasms, and severe headaches can result.

Person to person transmission is not frequent. Such transmission occurs only during pregnancy. Some six out of 1,000 women contract toxoplasmosis during pregnancy. Nearly half of these infections are passed on to the fetus. Congenital toxoplasmosis afflicts approximately 3,300 newborns in the United States each year. In such children, symptoms may be severe and quickly fatal, or may not appear until several months, or even years, after birth.

As for many other microbial diseases, the observance of good hygiene (including appropriate hand washing protocols) is a key means of preventing toxoplasmosis.