Tildesley and Keeling [30] examined whether R 0 was a good predictor of the 2001 UK Foot and Mouth disease. They concluded that R 0 explained just 29.3% of the standard deviation of the epidemic impact. They also noted that R 0 = 1 did not act as a threshold: at the value of R 0 = 1, only 20% of initial seedings generated epidemics; this probability increased to around 50% for the largest reasonable R 0 values. When heterogeneities exist in the population, infection is most likely to become focused within the high-risk individuals who are both more susceptible and more infectious. This highlights the stochastic nature of the disease in its early stages and the dependence of the ensuing epidemic on favourable local conditions in the neighbourhood of the initial infection.