Is it possible to prevent a major epidemic with an antiviral response that is started quickly after an introduction of the pathogen? To answer this question we performed stochastic simulations in which an antiviral response is initiated after a certain number of individuals are infected (see Text S1 for details). Figure 2 shows three representative simulation runs of an epidemic in a large but finite population (10 6 individuals). If no control measures are put into place (top panel), epidemiological theory informs that a large epidemic will unfold with probability 1{R {1 0 &0:67, and the fraction of individuals that is infected once the epidemic has taken off is roughly given by the solution of the final size equation (log(12x) = 2R 0 x) [15] . For the default parameter values this means that 94% of the population will be infected, of which some 2% will die from the sequelae of infection. In a population of one million individuals this implies that more than 18,000 individuals will die during the course of an epidemic.