In either case, having defined an appropriate populationlevel transmission rate, a stochastic susceptible-infectious model of transmission is defined through a Markov chain, in which a population with X susceptible individuals and Y infectious individuals transitions stochastically to a population with X − 1 susceptible individuals and Y + 1 infectious individuals at rate βXY/(N − 1). Then, the exact mean behaviour of such a system in the limit N → ∞ then has its transmission behaviour captured bẏ