ES-175

It's pretty easy to see that the number of people that a given infectious person infects is the product of1 the number of people they meet while infectious; the proportion of those people who are not immune; the chance of infecting each person. As the proportion if people who are immune goes up this number decreases, ultimately to below 1at which point the epidemic dies out more-or-less quickly: this is herd immunity. If this number is initially around 2, then herd immunity requires about h...