Cats are the definitive host in which sexual replication of Toxoplasma gondii takes place. Following replication of merozoites within enterocytes of the cat gut (a process known as merogony), male and female T. gondii gametes are formed within the host cell2. The fusion of gametes leads to the formation of diploid oocysts that are shed in cat faeces and undergo meiosis in the environment to yield eight haploid progeny sporozoites. In the intermediate host (shown here as rodents), asexual replication occurs. Acute infection is characterized by tachyzoites (rapidly replicating forms) that disseminate throughout the body. Differentiation to slow-growing bradyzoites within tissue cysts leads to long-term chronic infection. Ingestion of tissue cysts via omnivorous or carnivorous feeding can lead to transmission to either other intermediate hosts or to cats, re-initiating the sexual phase of the life cycle. Oocysts are capable of surviving in the environment for long periods of time, and sporulated oocysts (which are infectious) can contaminate food and water, providing a route of infection for intermediate hosts. Many animals serve as intermediate hosts, including farm animals. Humans become infected by eating undercooked meat containing tissue cysts or by ingesting sporulated oocysts in contaminated water4, 5. Although most human infections are mild, toxoplasmosis can cause serious symptoms in the brain and other organs (as indicated) in immunocompromised patients, as well as in the developing fetus following congenital infection.