The infectiousness of a disease is characterized by the basic reproduction number (R 0 ), which for our purposes is the expected number of infectious contacts per infective when there are no pharmaceutical or behavioural interventions in place and every individual is equally susceptible. More sophisticated definitions are required where individuals have substantially different risks of infection; the methods described by Diekmann & Heesterbeek (2000) are useful in defining and calculating R 0 when contact structures and other kinds of heterogeneity are important. In practice, when an epidemic occurs, the effective reproduction number (R) differs from R 0 due to the deployment of interventions, the build-up of herd immunity and possibly pre-existing immunity.