Back in September, when the West African Ebola outbreak was getting worse with every passing week, a lot of people began to worry that the virus could spread by air. And even if it couldn’t spread by air yet, they worried that it might be on the verge of mutating into an airborne form.

When I talked to virus experts, they saw little ground for either concern. The epidemiology of the outbreak, like previous ones, had the sort of pattern you’d expect from a virus that spreads mainly through contact with body fluids. A look at the evolutionary history of viruses indicates that a fluid-adapted virus would be unlikely to switch to going airborne with just a couple mutations. (I wrote