Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

Nearly all zoonotic diseases result from infection by one of six kinds of pathogen: viruses, bacteria, fungi, protists (a group of small, complex creatures such as amoebae, formerly but misleadingly known as protozoans), prions, and worms.

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.

“Spillover” is the term used by disease ecologists (it has a different use for economists) to denote the moment when a pathogen passes from members of one species, as host, into members of another.

They were right, and the virus became known as SARS coronavirus, inelegantly abbreviated as SARS-CoV. It was the first coronavirus ever found to inflict serious illness upon humans. (Several other coronaviruses are among the many viral strains responsible for common colds. Still others cause hepatitis in mice, gastroenteritis in pigs, and respiratory infection in turkeys.) SARS-CoV has no ominous ring. In older days, the new agent would have received a more vivid, geographical moniker such as Foshan virus or Guangzhou virus, and people would have run around saying: Watch out, he’s got Guangzhou! But by 2003 everyone recognized that such labeling would be invidious, unwelcome, and bad for tourism.

One thought that turns up in the latter sort is that “humankind has had a lucky escape.” The scenario could have been very much worse. SARS in 2003 was an outbreak, not a global pandemic. Eight thousand cases are relatively few, for such an explosive infection; 774 people died, not 7 million.

Lyme disease, psittacosis, Q fever: These three differ wildly in their particulars but share two traits in common. They are all zoonotic and they are all bacterial. They stand as reminders that not every bad, stubborn, new bug is a virus.

One of the signal lessons of Lyme disease, as Rick Ostfeld and his colleagues have shown, is that a zoonosis may spill over more readily within a disrupted, fragmented ecosystem than within an intact, diverse ecosystem.

“From the ecological point of view an outbreak can be defined as an explosive increase in the abundance of a particular species that occurs over a relatively short period of time.” Then, in the same bland tone, he noted: “From this perspective, the most serious outbreak on the planet earth is that of the species Homo sapiens.”

As of last week, I said, we’ve got 7 billion humans on this planet. It seems like an outbreak population. We live at high densities. Look at Hong Kong, look at Mumbai. We’re closely interconnected. We fly around. The 7 million people in Hong Kong are only three hours away from the 12 million people in Beijing. No other large animal has ever been as abundant. And we’ve also got our share of potentially devastating viruses.

I haven’t said much about influenza in this book, but not because it isn’t important. On the contrary, it’s vastly important, vastly complicated, and still potentially devastating in the form of a global influenza pandemic. The Next Big One could very well be flu.

Burke’s third criterion was “intrinsic evolvability,” meaning readiness to mutate and to recombine (or reassort), which “confers on a virus the potential to emerge into and to cause pandemics in human populations.” As examples he returned to retroviruses, orthomyxoviruses, and coronaviruses. “Some of these viruses,” he warned, citing coronaviruses in particular, “should be considered as serious threats to human health. These are viruses with high evolvability and proven ability to cause epidemics in animal populations.” It’s interesting in retrospect to note that he had augured the SARS epidemic six years before it occurred.

That’s the salubrious thing about zoonotic diseases: They remind us, as St. Francis did, that we humans are inseparable from the natural world. In fact, there is no “natural world,” it’s a bad and artificial phrase. There is only the world. Humankind is part of that world, as are the ebolaviruses, as are the influenzas and the HIVs, as are Nipah and Hendra and SARS, as are chimpanzees and bats and palm civets and bar-headed geese, as is the next murderous virus—the one we haven’t yet detected.