The paper I linked to above has as one hypothesis that it was a naturally occurring virus, cultures of which had been stored or worked on at the lab, and "leake

The paper I linked to above has as one hypothesis that it was a naturally occurring virus, cultures of which had been stored or worked on at the lab, and "leaked" by way of infection of workers there. Seems just as plausible as some source virus having been manipulated to increase its infectiousness to humans.