Despite their rough overlap in symptoms, flu viruses and the new coronavirus are biologically distinct in ways that might give the newcomer an edge. The coronavirus seems to transmit more readily to others, even from people without symptoms, and has been linked to more super-spreader events. Microscopic anatomical differences might make the coronavirus more likely to cling to airborne globs of spittle, or tougher to snuff out as it traverses the space among hosts. (Oddly, rhinoviruses, the main culprits behind the common cold, haven’t been stamped out to the same extent as the flu—a hardiness that could be attributed to their tough outer armor or an as-yet-unidentified quirk in how they spread among people.)

“Flu just tends to be a lot less transmissible, which means it’s easier to suppress it,” says Shweta Bansal, a disease ecologist at Georgetown University. “That’s partly why we’re getting away with an imperfect response right now.”

All these factors together might have cleared a path for the coronavirus to jostle its way to the top, at the expense of other viruses. That tussle might even be playing out directly in individual airways: People’s immune systems tend to remain on guard for a while after they’ve cleared an infection by one virus, bolstering them against immediate invasion by another. This balance of power won’t necessarily persist in the long term, though. Viruses that settle into a population may eventually learn to play nice with one another—perhaps becoming cordial roommates, setting up simultaneous infections in the same individual, even riling one another up.