The exponential growth-to-the-whole-population makes a major assumption: everyone is susceptible. Given the virulence of COVID-19, and the surprising lack of total population penetration in Hubei (now that we know just how late the quarantining efforts were given the apparent latency between carrier status and symptoms) one wonders if there isn't, say, a genetic factor which makes one more susceptible than others, for starters, and if COVID-19 simply has/will burn itself out in many places.