Sleeping sickness is an infectious disease caused by trypanosomes occurring in Africa and carried by tsetse flies. It causes fever, headache, often a sense of oppression, and a rash; later the characteristic somnolence follows and the disease enters a chronic, often fatal stage. Treatment is most effective if started before the late stage of brain involvement and uses arsenical compounds. There are two types of African trypanosomiasis, East and West, named for the region of Africa in which they were historically found.