HIV, AIDS, Syphilis, Herpes, Gonorrhea, Papilloma virus (which can cause cancer), Candida, Chlamydia, Trichomoniasis, Bacterial Vaginosis, ... all of these someone can bring back to their spouse/partner after straying into an affair. You never know who that person slept with and who they slept with, etc. I once heard someone explain STD awareness as realizing that you are having sex with everyone else that the other person has had sex with. (Its almost like a pyramid scheme). You don't know who all those other partners came into contact with.