Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

Sexual transmission is a good scheme for viruses with low hardiness in the external environment. It’s a mode of passage that doesn’t require them to go outside. They’re virtually never exposed to daylight or dry air. The virions pass from one body to another by way of direct, intimate contact between host cells lining delicate genital and mucosal surfaces. Rubbing (not just pressing) those surfaces together probably helps. Transmission during coitus is a conservative strategy, reducing risk to such viruses, sparing the need for hardening against desiccation or sunlight. But it has a downside too—notably, that opportunities for transmission are fewer. Even the most lubricious humans don’t have sex as often as they exhale. So the sexually transmitted viruses tend toward patience. They cause persistent infections and endure long periods of latency, punctuated by recurrent outbreaks (like herpesviruses); or else they replicate slowly (like HIV-1 and hepatitis B) up to a critical point at which things get bad. Such patience within a host gives the virus more time and therefore more sexual encounters by which to get itself passed along. Vertical transmission, meaning mother-to-offspring, is another slow, cautious mode. It can occur during pregnancy, during birth, or (in the case of mammals) by way of milk while an infant nurses. HIV-1, for instance, can be transmitted from mother to fetus across the placenta, or to a newborn in the birth canal, or through breastfeeding; but each...