Spillover: the powerful, prescient book that predicted the Covid-19 coronavirus pandemic.

by David Quammen

“Coevolution of Hosts and Parasites,” appeared in 1982. It began by dismissing those “unsupported statements” in medical and ecological textbooks19 “to the effect that ‘successful’ parasite species evolve to be harmless to their hosts.” Bosh and nonsense, said Anderson and May. In reality the virulence of a parasite “is usually coupled with the transmission rate and with the time taken to recover by those hosts for whom the infection is not lethal.” Transmission rate and recovery rate were two variables that Anderson and May included in their model. They noted three others: virulence (defined as deaths caused by the infectious agent), deaths from all other causes, and the ever-changing population size of the host. The best measure of evolutionary success, they figured, was the basic reproductive rate of the infection—that cardinal parameter, R0. So they had five crucial variables and they wanted to understand the net effect. They wanted to trace the dynamics. This led them to a simple equation. There will be no math questions in the quiz at the end of this book, but I thought you might like to cast your eyes upon it. Ready? Don’t flinch, don’t worry, don’t blink: R0 = βN/(α + b + v) In English: The evolutionary success of a bug is directly related to its rate of transmission through the host population and inversely but intricately related to its lethality, the rate of recovery from it, and the normal death rate from all other causes.