I don’t think that it was the most deadly epidemic as a percentage of population. That award probably goes to the Black Death. In Iceland, that ripped through the population twice, killing about half of the population each time.
Another oddity about the WWI flue is that it killed mostly young adults, not so much little children or the elderly. This has suggested to some that part of its deadliness is that it engendered an extreme iand dangerously excessive immune response.
I read a couple of books about it this summer. It sounds like it was an amazingly awful experience. What I find interesting is the lack of contemporaneous writings about it. It was as if it was too horrible an experience on which to reflect.
Makes sense. The danger in pneumonia is the immune response, the result of which is excessive fluid buildup in the lungs.
Even the Black Death probably doesn't qualify. Various virgin-field epidemics hit the no-immunity populations of the western hemisphere after the two disease ecosystems merged around 1500. It appears some of these had well over 50% mortality rates. In any case, the native population of the Americas had been reduced by 90 to 95% by 1600. Very few of those who died of these diseases ever saw a white man.
BTW, these are usually called European diseases. Almost all orignated in Asia, with a few coming from Africa. I don't know of any that actually originated in Europe.