Toxoplasmosis. Coloured transmission electron micrograph (TEM) of Toxoplasma gondii parasites inside a human cell. Infection with these parasitic protozoa causes the disease toxoplasmosis. This causes weeks of aching muscles and fatigue, but in extreme cases, or in immunocompromised people, it can lead to brain damage. The protozoa are found in many animals, and the majority of cases are caused by eating raw meat. The parasite's primary host is the cat, and it can also be contracted by handling cat faeces. The protozoa form cysts that are resistant to most treatments, although the antibiotic atovaquone is effective. Magnification: x5500 when printed 10cm high.