In the third scenario, a progenitor form of the virus had passed by spillover from an animal host into one or more persons, and then, during some period of slow, inefficient, unrecognized transmission among humans, acquired its nifty cleavage site by evolutionary steps that vastly improved its mediocre transmissibility. This period of sputtering, unnoticed human infection might have happened in October and November of 2019, possibly stretching back even earlier.
