Indeed H1N1 has popped up from time to time, with varying outcomes, not least as prior immunity waned. We are also much better at treating people, particularly

Indeed H1N1 has popped up from time to time, with varying outcomes, not least as prior immunity waned. We are also much better at treating people, particularly bacterial coinfections. Flu is a bit peculiar with viral RNA/DNA (I forget which) reassortment. However it did not fade out or transmute at the end of the 1918 pandemic due to a selection pressure to become less lethal (or at least, we have no evidence that that is the case)