A coronavirus typically gains mutations on a slow-but-steady pace of about two per month. But this variant, called B.1.1.7, had acquired 23 mutations that were not on the virus first identified in China. And 17 of those had developed all at once, sometime after it diverged from its most recent ancestor. Experts said there’s only one good hypothesis for how this happened: At some point the virus must have infected someone with a weak immune system, allowing it to adapt and evolve for months inside the person’s body before being transmitted to others.