where β is related to both the number of contacts an individual has with other individuals and the likeli- hood that an infected individual transmits the infection to a susceptible individual upon contact, γ is the rate at which infected individuals recover from an infection, which is taken as 1/(mean duration of the ill- ness), and N is the population size, assumed to be constant. The SIR model is termed a homogeneous mixing model because of three implicit assumptions: the population is fully mixed in such a way that a susceptible individual is equally likely to have contact with anyone in the population; all individuals have the same number of contacts per unit time; and infected individuals transmit the disease to any susceptible individual with the same probability. Each of these assumptions is relaxed in an agent-based epidemic model, which, unlike the deterministic system in Equation (1), is also stochastic in nature.