I once caught something nasty from my very x partner (nothing that wasnt cleared up witha bit of medication thank the stars!!) who cheated on me ALOT and obviously without protection, I was ***** scared to go have that STD test after we broke up, but I made myself do it! luckily I havent been left with anything "memorable" from him.
I get tested for chlamydia every pap smear (every year for me, family history of abnormal cells) I have had the same partner for 11 years now, but one just never knows, all it takes is just one slip up.

on another note, in my little bros group of friends its quite incestual the way they all sleep with eachother () if one gets somethng, guaranteed they all get it, I know for a fact that most of them have herpes now!