This CA model shows the spread of an epidemic through a grid of susceptible cells. Each infected cell affects its immediate neighbours which either become infected, or else become immune to infection. Epidemics display critical behaviour: the infection rate must be greater than a critical thresholds or the infection will not spread. The threshold depends on the size of the neighbourhood, so the critical infection rate is ~0.4 for 8 neighbours and ~0.59 for 4 neighbours. Behaviour is inherently unpredictable at the critical rate.