FIGURE 2. Expected pattern of spread of an uncontrolled epidemic.

a, Time sequence (in days) of an epidemic, showing spreading in a single simulation of an epidemic with R₀ = 1.5. Red indicates presence of infected individuals, green the density of people who have recovered from infection or died. b, Daily incidence of infection over time for R₀ = 1.5 in the absence of control measures. Thick blue line represents average for realizations resulting in a large epidemic, grey shading represents 95% confidence limits of the incidence time series. Multiple coloured thin lines show a sample of realizations, illustrating a large degree of stochastic variability. c, Root mean square (r.m.s.) distance from the seed infective for individuals infected since the start of the epidemic as a function of time. Thick blue line represents average distance for realizations resulting in a large epidemic, grey shading represents 95% limits. d, Proportion of the population infected by age for R₀ = 1.5, averaged across realizations that result in large epidemics. The infection attack rate is 33% for R₀ = 1.5 and 50% for R₀ = 1.8. e, Distribution of the number of secondary cases per primary case during the exponential growth phase of a R₀ = 1.5 epidemic. Between 50 and 1,000 realizations were used to calculate all averages (see Supplementary Information).