" increases of the infectivity rate may lead to large epidemics.” This quiet warning has echoed loudly ever since. It’s a cardinal truth, over which public health officials obsess each year during influenza season. Another implication was that epidemics don’t end because all the susceptible individuals are either dead or recovered. They end because susceptible individuals are no longer sufficiently dense within the population. W. H. Hamer had said so in 1906, remember? Ross had made the same point in 1916. But the paper by Kermack and McKendrick turned it into a working principle of mathematical epidemiology. "

― David Quammen , Spillover: Animal Infections and the Next Human Pandemic