When scientists find a new disease-causing virus in humans, the first question they want answered is: Can this thing spread easily from person to person? And if not, will it someday?
Avian influenza can infect humans, but it’s not very good at passing from one person to another. Same with the coronavirus that causes MERS, another severe respiratory illness. Though hundreds of cases of both have been reported over the past two decades, neither virus seems inclined to evolve toward efficient person-to-person transmission.