Sleeping sickness is caused by trypanosomes, which are protozoan parasites that are delivered to your blood stream by tsetse fly (Glossina Genus) bites2. These dangerous tsetse flies live in Africa and can be found by rivers, lakes, forests, and savannahs2. There is actually two parasite subspecies that cause trypanosomiasis infection known as; trypanosoma brucei rhodesiense (T.b.r.), which causes acute illness lasting several weeks in countries of eastern and southern Africa and trypanosomoa brucei gambiense (T.b.g.), which causes a chronic infection lasting years and affecting countries of western and central Africa3. T.b.g. is the most common because it is found in 95% of sleeping sickness cases, compared to the other parasite and the T.b.g. needs two hosts in order to live and reproduce, and its life cycle starts once an infected tsetse fly bites human skin5. The two hosts are an insect vector and mammalian host and due to the large difference between these hosts the trypanosome undergoes complex changes during its life
References: 8. Kennedy, P.G.E. 2004. Human African trpanosomiasis of the CNS: current issues and challenges. J. Clin. Invest 113:496-504.