Yes, it was the second wave that was contrary to most flu epidemics. The current theory is that people in the trenches with a mild case stayed put, while the severe cases were sent to field hospitals, where it was spread further.

But, I was referring to why normally healthy people were vulnerable. The "cytokine storm" theory was developed relatively recently, after researchers transfected animals with the virus from frozen victims, and they suffered rapid progressive respiratory failure.

I can't find the article now, but yesterday I read one that reported some of the younger patients in Italy fit this model.