Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

Transmission is travel from one host to another, and transmissibility is the packet of attributes for achieving it. Can the virions concentrate themselves in a host’s throat or nasal passages, cause irritation there, and come blasting out on the force of a cough or a sneeze? Once launched into the environment, can they resist desiccation and ultraviolet light for at least a few minutes? Can they invade a new individual by settling onto other mucous membranes—in the nostrils, in the throat, in the eyes—and gaining attachment, cell entry, another round of replication? If so, that virus is highly transmissible. It goes airborne from one host to another. Fortunately, not every virus can do that. If HIV-1 could, you and I might already be dead. If the rabies virus could, it would be the most horrific pathogen on the planet. The influenzas are well adapted for airborne transmission, which is why a new strain can circle the world within days. 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.