Which stage of the Toxoplasma gondii lifecycle is infectious to humans?

Toxoplasma gondii is an intracellular parasite that can infect a range of hosts, including humans. The infectious stage for humans primarily involves oocysts. These oocysts are shed in the feces of infected cats, the definitive hosts of the parasite.

When humans come into contact with contaminated soil, water, or food, or if they inadvertently ingest oocysts, they can become infected. This stage is particularly resilient and can survive in the environment for extended periods, making it a significant route of infection.

While trophozoites, cysts, and sporozoites are all stages in the lifecycle of Toxoplasma gondii, they do not represent the primary transmission route to humans. Trophozoites are often not found in environmental samples, and cysts, although they can be infectious, usually represent a more advanced stage that comes from consuming tissue from infected animals, typically undercooked meat. Sporozoites are specifically the form present within oocysts but do not represent the infectious form when considering how humans are commonly infected through oocysts. Thus, oocysts stand out as the key infectious stage for humans in the lifecycle of Toxoplasma gondii.