African sleeping sickness is caused by two different subspecies of the parasite Trypanosoma brucei, T. b. gambiense and T. b. rhodesiense. Humans are the primary reservoir for the parasite; the only known vector is the tsetse. The clinical manifestations of infections caused by both subspecies of the pathogen overlap, but the timeline for the progression of the illness is very different between the two forms. In recent years, the epidemics of sleeping sickness that were once widespread across much of Sub-Saharan Africa have been reduced to small outbreaks. Currently, less than 700 cases occur each year with half of those cases being reported from the Democratic Republic of Congo. Approximately 85% of cases are now caused by T. b. gambiense, the agent of West African sleeping sickness. Sporadic infections caused by the less common T. b. rhodesiense subspecies are restricted to eastern and southern Africa.