Summary A definition of an arbovirus and a broad idea of the groups and the numbers isolated and causing human infection are given. The small incidence of clinical disease compared with overall infection rates is stressed. The conditions for the successful survival of arboviruses is outlined. The investigation of the illness and origin of infection is described. The role of viral antibody in the development of encephalitis and the use of cortisone in treatment is discussed. The over-wintering of arboviruses and their capacity for latency is considered in relationship to the perpetuation of virus and the pathogenic effects on the hosts involved.