1. Each individual is set as infectious with probability I Ã (and conversely, the probability of being set susceptible is 1{I Ã ). 2. A random ½S{I pair is chosen, and the susceptible individual is infected. 3. A random infectious individual is placed into the susceptible class. 4. Steps 2 and 3 are repeated until spatial structure is equilibrated, and then averages ½I and ½S{I of the number of infectious individuals and susceptible-infectious pairs are taken over a further set of iterations of 2 and 3. 5. The transmission rate is then given by t Ã~g ½I ½S{I .