What if, around the time the black plauge hit europe in OTL, instead the Spanish Influenza arrives in europe
Living to 60 wasn't out of the question in the middle ages, even for peasants. It's taken as true on this site that modern people can't go back in time because their immune systems can't handle the regular pathogens humans back then dealt with daily. So if human immune systems were stronger then, that means the flu will be more deadly. And it does take a while to incubate, 7-10 days so you could get pretty far. I think you're going to get significant depopulation but I'm not sure how many. H5N1 has 60% kill rates, H1N1 had lower than usual unless you had some other issue like asthma. The exception to that is Ukraine which apparently got a variant that was more deadly.To get this out before it gets outta hand: No one expects the Spanish Influenza!
Now, from a quick wiki look, the virus worked by creating an overreaction in the immune system, which is why the robust immune system of young adults killed them, while younger and older people (with their weaker immune systems) were more apt to survive. So perhaps this creates a lost generation, allowing the young and older to survive, but killing off the middle? And as people in the middle ages didn't live to be that old anyway....