These are unusual parasites among the apicomplexans because they have two vertebrate hosts in the life cycle. They infect epithelial cells of the intestines of cats and their relatives, and then produce a resistant cyst that infects various herbivores, including humans. There, they develop in the musculature and can infect a cat, or even humans, feeding on contaminated flesh. Toxoplasmosis takes a number of forms, depending on the location and age of infection. The most serious is congenital toxoplasmosis, which occurs when a female contracts the parasite during pregnancy and the fetus becomes infected. Although often asymptomatic, it can cause abortion or birth defects, in humans or in other animals.