What I am thinking might be happening here is not that people have been infected for some time with this (SARS-CoV-2) virus, but that it is finding a new niche rather more slowly and that could ultimately cause more of a problem than we have seen with other diseases because it is not so spectacular early on in its evolution.
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Trevor Drew (2020) cited in: "Virologist heading coronavirus vaccine efforts warns the killer disease could mutate and become even deadlier - as Qantas jet waits on standby to fly stranded Australians out of Wuhan" in Daily Mail, 31 January 2020.