Pathol. 1. Also called African sleeping sickness, African trypanosomiasis. a generally fatal disease, common in parts of Africa, characterized by fever, wasting, and progressive lethargy: caused by a parasitic protozoan, Trypanosoma gambiense or T. rhodesiense, that is carried by a tsetse fly, Glossina palpalis. 2. Also called epidemic encephalitis, lethargic encephalitis. a viral disease affecting the brain, characterized by apathy, sleepiness, extreme muscular weakness, and impairment of vision. Also,Brit.,sleepy sickness.