If blood type O was predominant in the pre-plague era and blood type A was predominant after, could we say that A was selected for via the plague.
Statistically, O is stronger against heart disease and cancer, but weak for anything that came from Pandora's Box. A's die like flies even today, and at younger ages, from cancer and heart disease.
Fox news last night had a mention of the French, who despite their diet had a lot of longevity. France should have been wiped out by plague, but there were pockets that refused outsiders entry. None died. They would still be heavily O’s.
It's more like a series of diseases, some viral in nature and others bacterial, require some point of attachment on cell surfaces that simply isn't there in some people. This theory has been studied to some degree and the diseases identified. There are people immune to both black plague and cholera, and they appear to be immune to several other quite deadly but common infectious diseases.
Alas, none of the studies have been able to pin down the cause of this immunity (which includes an immunity to AIDs).