The o'nyong'nyong virus (ONNV) was first isolated by researchers at the Uganda Virus Research Institute in Entebbe, Uganda, during a large outbreak of a disease in 1959 that resembled dengue fever. ONNV is a togavirus (family Togaviridae), genus Alphavirus, is closely related to the chikungunya and Igbo Ora viruses, and is a member of the Semliki Forest antigenic complex. The name was given to the disease by the Acholi tribe during the 1959 outbreak. The name comes from the Nilotic language of Uganda and Sudan and means “weakening of the joints". The virus can infect humans and may cause disease.