I would rather inject myself with AIDS than stick some strangers disease riddled regulator in my mouth.
This is beyond the pall; don't even say this! AIDS is a potentially fatal disease.
I have buddy breathed in many different situations, and never gotten sick from it. For Pete's sake, during the early decades of diving, this was the only method of sharing air (1945 to about 1975).
For my NAUI ITC in 1973, we instructor candidates had to do in-water mouth-to-mouth on each other while towing the "victim" through 200 yards of surf. I lost one of my Jet Fins, but completed the exercise. It was very instructive as to just how effective properly done mouth-to-mouth resuscitation actually is.
I have gotten very sick from diving in polluted water though. As a research diver for the Oregon Department of Fish and Wildlife doing subtidal clam bed surveys, I incidentally drank some Tillamook Bay water that was apparently contaminated with sewage. That evening, I could not figure out which end of me to point at the toilet in the motel room--very bad situation, but I recovered.
My take-away:
--don't be overly concerned about sharing someone's mouthpiece/second stage in an out-of-air situation (simulated or real).
--don't dive in sewage-contaminated water (heed the official warnings).
--don't equate either situations with a potentially fatal disease.
SeaRat