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

by David Quammen

What Walsh suggested—to recapitulate in simplest form—is a wave of Ebola virus sweeping across Central Africa by newly infecting some reservoir host or hosts. From its recent establishment in the host, according to Walsh, the virus spilled over, here and there, into ape and human populations. The result of that process is manifest as a sequence of human outbreaks coinciding with clusters of dead chimps and gorillas—almost as though the virus were sweeping through ape populations across Central Africa. Walsh insisted during our Libreville chat, though, that he had never proposed a continental wave of dying gorillas, one group infecting another. His wave of Ebola, he explained, has been traveling mainly through the reservoir populations, not through the apes. Ape deaths have been numerous and widespread, yes, and to some degree amplified by ape-to-ape contagion, but the larger pattern reflects progressive viral establishment in some other group of animals, still unidentified, with which apes frequently come into contact. Leroy, on the other hand, has presented his particle hypothesis of “multiple independent introductions” as a diametric alternative not to Walsh’s idea as here stated but to the notion of a continuous wave among the apes. In other words, one has cried: Apples! The other has replied: Not oranges, no! Either might be right, or not, but in any case their arguments don’t quite meet nose to nose.