Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

The oral-fecal route sounds disgusting but is really quite common. It works well for some viruses because host creatures (including humans) are often forced, especially when living at high densities, to consume food or water contaminated by excrement from other members of their population. This is one of the reasons why children die of dehydration in rainy refugee camps. The virus goes in the mouth, replicates in the belly or the intestines, causes gastrointestinal distress, may or may not spread to other parts of the body, and comes gushing out the anus. Diarrhea, for such a virus, is part of an effective strategy for dispersal. Viruses transmitted this way tend to be fairly hardy in the environment, because they may need to linger in that polluted sump for a day or two before some desperate person comes to drink from it. There’s an entire group of such viruses, known as the enteroviruses, including polio and about seventy others, that attack us in the gut. Most of those enteroviruses are uniquely human infections, not zoonoses. Evidently they don’t need other animal hosts for maintaining themselves in a crowded human world. For blood-borne viruses, transmission is more complicated. Generally it depends on a third party, a vector. The virus must replicate abundantly in the blood of the host to produce severe viremia (that is, a flood of virions). The vector (a blood-sucking insect or some other arthropod) must arrive for a meal, bite the host, slurp up virions along with ...