Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

Infectious disease is all around us. Infectious disease is a kind of natural mortar binding one creature to another, one species to another, within the elaborate biophysical edifices we call ecosystems.

An amplifier host is a creature in which a virus or other pathogen replicates—and from which it spews—with extraordinary abundance.

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. There are three elements to the situation. One: Mankind’s activities are causing the disintegration (a word chosen carefully) of natural ecosystems at a cataclysmic rate.

Students of virology now speak of the “virosphere,” a vast realm of organisms that probably dwarfs every other group.

A parasitic microbe, thus jostled, evicted, deprived of its habitual host, has two options—to find a new host, a new kind of host . . . or to go extinct. It’s not that they target us especially. It’s that we are so obtrusively, abundantly available.

“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.

Gorillas can be exposed to such infections wherever unhealthy people are walking, coughing, sneezing, and crapping in the forest. Any such spillover in the reverse direction—from humans to a nonhuman species—is known as an anthroponosis.

This is a pointed example of why solving the reservoir mystery is important: If you know which animal harbors a certain virus and where that animal lives—and conversely, where it doesn’t live—you know where the virus may next spill over, and where it probably won’t.

Sheer lethality may be irrelevant to the virus’s reproductive success and long-term survival, the measures by which evolution keeps score. Remember, the human body isn’t the primary habitat of ebolaviruses. The reservoir host is.

Like other zoonotic viruses, ebolaviruses have probably adapted to living tranquilly within their reservoir (or reservoirs), replicating steadily but not abundantly and causing little or no trouble.

Some scientists use the term “dead-end host,” as distinct from “reservoir host,” to describe humanity’s role in the lives and adventures of ebolaviruses.

Outbreaks have been contained and terminated; in each situation the virus has come to a dead end, leaving no offspring. Not the virus in toto throughout its range, of course, but that lineage of virus, the one that has spilled over, betting everything on this gambit—it’s gone, kaput. It’s an evolutionary loser. It hasn’t caught hold to become an endemic disease within human populations.

Advisory: If your husband catches an ebolavirus, give him food and water and love and maybe prayers but keep your distance, wait patiently, hope for the best—and, if he dies, don’t clean out his bowels by hand. Better to step back, blow a kiss, and burn the hut.

They believed in a form of malign spirit, called gemo, that sometimes swept in like the wind to cause waves of sickness and death.

Once a true gemo was recognized, as distinct from a lesser spate of illness in the community, Acholi cultural knowledge dictated a program of special behaviors, some of which were quite appropriate for controlling infectious disease, whether you believed it was caused by spirits or by a virus. These behaviors included quarantining each patient in a house apart from other houses; relying on a survivor of the epidemic (if there were any) to provide care to each patient; limiting movement of people between the affected village and others; abstaining from sexual relations; not eating rotten or smoked meat; and suspending the ordinary burial practices, which would involve an open casket and a final “love touch” of the deceased by each mourner, filing up for that purpose. Dancing was also prohibited. Such traditional Acholi strictures (along with intervention by the Uganda Ministry of Health and support from the CDC, Médecins Sans Frontières, and WHO) may have helped suppress the Gulu outbreak.

“We have a lot to learn from these people,” Barry Hewlett told me, one day in Gabon, “as to how they’ve responded to these epidemics over time.” Modern society has lost that sort of ancient, painfully acquired accumulation of cultural knowledge, he said. Instead we depend on the disease scientists. Molecular biology and epidemiology are useful, but other traditions of knowledge are useful too.

Ebola virus generally killed with a whimper, not with a bang or a splash.

This elaborate concatenation of life-forms and sequential strategies is highly adaptive and, so far as mosquitoes and hosts are concerned, difficult to resist. It shows evolution’s power, over great lengths of time, to produce structures, tactics, and transformations of majestic intricacy. Alternatively, anyone who favors Intelligent Design in lieu of evolution might pause to wonder why God devoted so much of His intelligence to designing malarial parasites.

Malaria exemplifies this. Within the Plasmodium family tree, as revealed by molecular phylogenetics over the last two decades, the four human-afflicting kinds don’t cluster on a single branch. They are each more closely related to other kinds of Plasmodium, infecting nonhuman hosts, than to one another. In the lingo of taxonomists, they are polyphyletic. What that suggests, besides the diversity of their genus, is that each of them must have made the leap to humans independently.