a, Schematic illustration of the time course of an acute (short-duration) immunizing viral infection passing through a population, such as occurs in FMD outbreaks among livestock. b, The epidemic is captured by a simple SIR model of disease dynamics; proportions of susceptible (orange), infected (red) and recovered, seropositive individuals (blue) are presented relative to the initial number of exposed susceptible individuals. During the epidemic, timely surveillance of incidence and viral genetics can give rich information on epidemic dynamics and evolution. However, this opportunity is brief, as the epidemic rapidly extinguishes itself by depleting its susceptible ‘fuel’. By contrast, the epidemiological signal from serological studies of recovered, seropositive individuals is much more persistent, declining on the much slower timescale of population turnover.