Transitions in the simulation are decided by a stochastic process. The simulation starts with N wild-type infectives. Each day each infective has its infectious age increased by 1. A random number in the interval (0,1) is then generated for each infective. If this number is ,current recovery probability b i then the individual recovers, otherwise it generates its quota of secondary infectives m i . New infectives begin with infectious age = 0. For each new infective a random number in the range (0,1) is generated. If it is ,mutation rate then this new infective will be a variant strain. Mutations are generated by a small perturbation to the function V i (see below ''Mutation and Competition''). Each day the number of each strain is calculated so that the dominant (highest frequency) strain can be identified. The entropy, S, of this strain is then calculated using the demographic parameters.