The genotypes of mutant and wild-types are constructed so as to have positive growth rates. Consecutive time-steps of evolution are simulated by generating random numbers to decide which individuals recover or continue as infectives. To simulate competition between strains some additional growth constraints have to be applied to each scenario. In the Ww0 case the initial population is allowed to grow rapidly from an initial starting number N init . Following exhaustion of the supply of susceptibles (i.e. resources are depleted) an extrinsic mortality 1{N init =N ð Þis applied to all individuals to reduce the population of infectives back down to its starting value. This process is repeated over many time-steps. In the Wv0 case if the total population size (N max ) of infectives is exceeded an extrinsic mortality 1{N max =N ð Þis applied probabilistically so that the population is maintained at a level that fluctuates around N max .