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

by David Quammen

The ultimate product of MacDonald’s equations was a single number, which he called the basic reproduction rate. That rate represented, in his words, “the number of infections distributed in a community13 as the direct result of the presence in it of a single primary non-immune case.” More precisely, it was the average number of secondary infections produced, at the beginning of an outbreak, when one infected individual enters a population where all individuals are nonimmune and therefore susceptible. MacDonald had identified a crucial index—fateful, determinative. If the basic reproduction rate was less than 1, the disease fizzled away. If it was greater than 1 (greater than 1.0, to be more precise), the outbreak grew. And if it was considerably greater than 1.0, then kaboom: an epidemic. The rate in Ceylon, he deduced from available data, had probably been about 10. That’s very high, as disease parameters go. Plenty high enough to yield a severe epidemic. But it was the lower side of the range for circumstances such as those in Ceylon. On the upper side, MacDonald imagined this: that a single infected person, left untreated and remaining infectious for eighty days, exposed to ten mosquitoes each day, if those mosquitoes enjoyed reasonable longevity and reasonable opportunities to bite, could infect 540 other people. Basic reproduction rate: 540.

I wonder what it's like for COVID