The question in my mind is why have so many young and healthy people died in China, like the Dr. that first raised the alarm there and was arrested? He should have had plenty of medical resources available to him - if he wasn't sitting in a prison cell.
There are as many factors as we do not have answers to our questions.
It appears victims are young and healthy, but living in an environment where air pollution tends to be incredible by American standards. Healthy, in that environment is relative.
Add to that questionable medical care,
possibly influenced by political factors, especially considering he was arrested.
Initial infections, if lab 'escapes', may have had an initial viral load far in excess of that normally acquired through ordinary communication means, overwhelming the immune system, rather than it having the ordinary time to ramp up if the initial infection had been a few virii.
It is also possible that the earliest infections involved a virus which has evolved somewhat to be more communicable but less lethal (a common trade off with viruses).
Now, these are speculation, I am not a medical doctor, just an old EMT who has done a little reading here and there.