There was an article in The NY Times a few days ago about how the rates of STDs are rising enormously amongst older Americans. I would suspect that many of these older folks are either widowed or divorced, and spent years fucking bare with their spouses. Now that they are hooking up with new people, and probably thinking pregnancy is out of the question, they go sans condoms, and there starts the outbreaks.
And the article mentions how the immune system weakens as we age, making the oldsters even more susceptible to infections and diseases