Toxoplasmosis is caused by a parasite, called Toxoplasma gondii.

After ingestion, toxoplasmosis eggs grow and release thousands of adult parasites. These organisms spread to other parts of the body, where they form tiny cysts in the muscles, lymph nodes, placenta and central nervous system. Toxoplasmosis can easily spread into the fetus during pregnancy.

A person with a healthy immune system does not develop symptoms, because the body inactivates the organism. The cysts can develop into an active infection in a person with a weak immune system.