Theileria species are protozoan parasites infecting wild and domestic animals in most tropical and subtropical regions of the world. Theileriosis is a general term used for infections in cattle with one or more of a number of Theileria species. Some of these may cause only mild or subclinical disease in cattle (so-called benign theileriosis), whilst others are extremely pathogenic, resulting in high mortality and severe economic losses. By far the most important and pathogenic Theileria species affecting cattle in Africa south of the Sahara is Theileria parva, which is responsible for the disease syndromes often referred to as East Coast fever, Corridor disease or Zimbabwe theileriosis.