Breathless: The Scientific Race to Defeat a Deadly Virus

by David Quammen

In the lingo of disease ecologists: a sylvatic cycle. That term comes from the Latin word sylva, meaning forest. A virus with a sylvatic cycle is two-faced, like a traveling salesman with another wife and more kids in another town. Yellow fever virus, for example: transmitted by mosquitoes, it infects humans in cities (the urban cycle) when the right mosquitoes are present, but it’s broadly enough adapted to infect monkeys also, and it does that in some tropical forests (the sylvatic cycle), circulating in monkey populations. Yellow fever can be eliminated in cities by vaccination and mosquito control, but whenever an unvaccinated person goes into a forest where the virus circulates, that person can become infected, return to the city, and trigger another urban cycle, if some mosquitoes are still there to help. Yellow fever virus has never been eradicated, and travelers to many tropical countries are still obliged to be vaccinated, because the sylvatic cycle will persist, and threaten another urban cycle, until you kill every mosquito or vaccinate every monkey.