They have done studies which indicate different individuals are more/less susceptible to certain viruses based on genetics. For example, some studies have indicated that blood type makes people more or less susceptible. As an example, type O might be less susceptible to Covid, but more susceptible to norovirus.

I think it would be interesting if this turned out to be true for something like blood type where individuals may express one type, but may also carry genes for other types.

Having different genes in the gene pool would allow some members of the group to survive a certain pathogen because they were less susceptible because they expressed that gene(s) while carrying the gene(s) which would allow for less susceptibility to other viruses. This would allow the survivors to repopulate with the full gene package.