title: Periodic solutions of an epidemic model.
abstract: The existence of periodic solutions of the equation (see formula in text) is established. This equation arises in the study of the spread of a disease which does not induce permanent immunity.
mesh_terms: D004198:Disease Susceptibility; D018562:Disease Transmission, Infectious; D004812:Epidemiologic Methods; D006801:Humans; D007109:Immunity; D008954:Models, Biological; D013997:Time Factors