Medical Definition of sleeping sickness
1: a serious disease that is prevalent in much of tropical Africa, is marked by fever, headache, protracted lethargy, confusion, sleep disturbances, tremors, and loss of weight, is caused by either of two trypanosomes (Trypanosoma brucei gambiense and T. b. rhodesiense), and is transmitted by tsetse flies—called also African sleeping sickness
2: any of various viral encephalitides or encephalomyelitides of which lethargy or somnolence is a prominent feature; especially : equine encephalomyelitis
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