One of the points that I feel that you are overlooking or not answering with the flu pandemic of the early 20th century that the auto immune over reaction deaths of other wise non immuo deficient populous.
Well, no, because I don't have any real knowledge of Spanish flu apart from reading a couple of books so I'm not equipped to agree on facts. Was just making the point that experts in the field are still divided on the detail.
As I said, 100 years on and the debate among medical professionals is still continuing.