We picked up scabies this last summer and I had to tell a playmate.
I still don't know if he and his wife are pissed off at us over that. While it's not an STD, it can be passed from intimate contact or by sharing linens (which would have happened that night even if nothing sexual had occurred). I felt like crap telling him, but *not* telling would have been the wrong thing to do IMO.
So, in my opinion, if either of us had something that was highly communicable, I would tell anyone we have played with during the time that it would have been infectious. It sucks, it really does, makes one feel dirty IME, but it's the right thing to do.