Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

To put the matter in its starkest form: Human-caused ecological pressures and disruptions are bringing animal pathogens ever more into contact with human populations, while human technology and behavior are spreading those pathogens ever more widely and quickly.

In some 25 or 27 years, we have doubled in number. A marvelous target for any organism that can adapt itself to invading us.”

As humans have increasingly entered the Bornean forests—killing and displacing macaques, cutting timber, setting fires, creating massive oil-palm plantations and small family farm plots, presenting themselves as an alternative host—both the necessity and the opportunity have increased.

“I honestly believe we’re at a sort of critical point. And we should be watching. We should be watching the situation very, very carefully,” she said. “And hopefully nothing will happen.” But of course, as she well knew, something always does happen. It’s just a question of what and when.

We are a relatively young kind of primate, we humans, and therefore our diseases are young too.

Some of those infections, such as Hendra and Ebola, visit us only occasionally and, when it happens, arrive soon at dead ends. Others do as the influenzas and the HIVs have done—take hold, spread from person to person, and achieve vast, far-flung, enduring success within the universe of habitat that is us.

One further factor, possibly the most crucial, was inherent to the way SARS-CoV affects the human body: Symptoms tend to appear in a person before, rather than after, that person becomes highly infectious.

If SARS had conformed to the perverse pattern of presymptomatic infectivity, its 2003 emergence wouldn’t be a case history in good luck and effective outbreak response. It would be a much darker story.

This case and similar ones, Burnet wrote, embodied a general truth about infectious disease. “It is a conflict between man and his parasites which, in a constant environment, would tend to result in a virtual equilibrium, a climax state, in which both species would survive indefinitely. Man, however, lives in an environment constantly being changed by his own activities, and few of his diseases have attained such an equilibrium.”

The SARS virus travels this route too, or anyway by the respiratory droplets of sneezes and coughs—hanging in the air of a hotel corridor, moving through the cabin of an airplane—and that capacity, combined with its case fatality rate of almost 10 percent, is what made it so scary in 2003 to the people who understood it best.

One in every four species of mammal is a bat.