The flu virus is very tricky in that it apparently has the ability to mutate extensively (as Douglas Adams once wrote: "over lunch") while remaining, well, flu. In the great flu epidemic, excuse me, pandemic of 1918, the described ease of transmission, rapidity of progression of symptoms, and death toll would read more symptomatic of the ebola of this day and age. But it was always flu. And more to the point, in that incarnation the death toll was higher for robust adults than for the very young and very old. There were reasons for this involving previous exposure to waves of virus over the years. There are very smart people who have devoted their lives to flu and they are worth listening to.