Swine Flu?
I agree with you, it may very well be a weaponized variant that got loose. Recall the H1N1 outbreak here had never been previously detected in pigs or birds. I don’t seem to recall if they ever did figure out where the variant came from? They did start vaccinating our military personnel at the time for it though.
I actually caught it. It was one of the worst illnesses I’ve ever had.