How Does A (COVID-19) Corona Virus Shift From Zoonotic To Human-To-Human Transmission?
When a virus passes from a nonhuman animal into a human, we call that moment of spillover a zoonotic transmission. It’s an ecological event. What happens next depends on evolutionary potential and chance. If the virus is adaptable, it may succeed in replicating and proliferating in the new human host. Maybe it kills the person and the line of transmission comes to an end there—as happens with rabies. But if the virus is even more adaptable, it may acquire the ability to pass from one human host to another, perhaps by sexual contact (as with HIV), perhaps in bodily fluids such as blood (as with Ebola), perhaps in respiratory droplets launched by coughing or sneezing (as with influenza or SARS). What makes a virus adaptable? The changeability of its genome, plus Darwinian natural selection. Those viruses with single-stranded RNA genomes, which replicate themselves inaccurately and therefore have highly changeable genomes, are among the most adaptable. Coronaviruses belong to that group.
David Quammen is the author of more than a dozen books, including Spillover: Animal Infections and the Next Human Pandemic, and hundreds of articles for publications including National Geographic, The Atlantic, Harper’s, Rolling Stone, and many others.