Zahavasdad………..
I believe you mean GENETIC diseases, not generic (such as a virus).
I have a close friend whose grandparents were first cousins and great-great grandparents were also first cousins. She and all the females in her generation have toes missing bones. They are also carriers of the Fragile X gene causing autism and mental retardation. All of their children are afflicted with these disorders..some mild and some requiring them to be kept in institutions.
And if you wonder why I have a close friend who is female, we almost married years ago, but when the premarital genetic testing was done the fact that she was a carrier came out and she decided she would not bear children and to remain single. She is a great Tzedakis, caring for assorted afflicted nieces and nephews giving her sisters and brothers-in-law needed respite.