I would say, as per the medical genius Trump--after all he knew it was airborne
back in Jan (and could have saved us all!) whereas the CDC just realized that
now--that viruses often 'just go away'.
It's a combination of resistance in the population and 'the dose makes the
poison'.
The thinking at the start was that there was no resistance to this 'novel' virus.
That is clearly not true and there is a large percentage of the population with
at least some resistance.
Since the dose makes the poison, resistance doesn't mean immune; if you inject
enough live virus into someone they are going to die. On the other hand, if
you inject a small enough amount, they will get an asymptomatic case and some
more resistance.
So children, for example, are pretty resistant to this: unlikely to get it, if they do get it
will most likely be asymptomatic, and they are extremely unlikely to pass it on.
Child to child is transmission is exceedingly rare because of that (small dose to
resistant recipient).
So when the virus first moves in to a region the most susceptible are going to get it first
and pass it along among themselves, which is the most likely to generate
deadly cases (big doses of virus into the less resistant). Once that least resistant
population has basically been worked through, further outbreaks are going to be less
severe in terms of symptoms--it just goes away.
Spain and France had severe first outbreaks, covering, I believe good percentages
of the country. So any further outbreaks are going to be less deadly.
The opposite example would be Australia. They had a first, very small outbreak. Their
second, recent outbreak has been at least as deadly, case percentage wise, as the
first.