Epidemic size distribution.

<p>Figure A illustrates the final epidemic size distributions <i>P</i>(<i>E</i>) for various values of <i>r</i>₀ in the stochastic SIR model. The corresponding result for the deterministic SIR model (a number <b>E</b>) is marked in each case by a dashed line. The left vertical scale in Figure A describes the small terminal infections (left of the first horizontal scale break), and the right vertical scale describes large-scale infections (right of the first horizontal scale break). Figure B illustrates the corresponding cumulative probability distribution <i>P</i>(<i>E</i> < <i>E</i>_max) or the probability of an epidemic of size less than <i>E</i>_max. This also shows the relative weight in the terminal infection and in the large-scale epidemic. In each case <i>N</i> = 500 and <i>I</i>₀ = 1.