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

by David Quammen

not every virus is “a piece of bad news wrapped up in a protein”—or at least, it’s not bad news for every host infected. Sometimes the news is merely neutral. Sometimes it’s even good; certain viruses perform salubrious services for their hosts. “Infection” need not always entail any significant damage; the word merely means an established presence of some microbe. A virus doesn’t necessarily achieve anything by making its host sick. Its self-interest requires just replication and transmission. The virus enters cells, yes, and subverts their physiological machinery to make copies of itself, yes, and often destroys those cells as it exits, yes; but maybe not so many cells as to cause real harm. It may inhabit a host rather quietly, benignly, replicating at modest levels and getting transmitted from one individual to another without producing any symptoms. The relationship between a virus and its reservoir host, for instance, tends to involve such a truce, sometimes reached after long association and many generations of mutual evolutionary adjustment, the virus becoming less virulent, the host becoming more tolerant. That’s in part what defines a reservoir: no symptoms. Not every virus-host relationship evolves toward such amicable relations. It’s a special form of ecological equilibrium.