**The** progenitor virus itself was almost certainly one that circulates harmlessly in bats (as SARS does) but has an “intermediate reservoir” in one or more animals that come into contact with people, Andersen said. Presumably, that reservoir is one of the species of animals at the Wuhan market thought to be ground zero for the outbreak. The ancestor of 2019-nCoV existed in that species for some unknown time, never infecting people, until by chance a single virus acquired a mutation that made it capable of jumping into and infecting humans.
The genome sequences suggest that was a one-time-only jump. “The genomes [from the 24 samples] are very uniform,” Andersen said. “If there had been multiple introductions,” including from many different animals, “there would be more genomic diversity. This was a single introduction.”
That means that what’s sustaining the spread is human-to-human transmission (suggesting that closing Wuhan’s animal market is very much an after-the-horse-has-fled-the-barn reaction).
