Provided the guy was really kidding about being HIV positive, I applaud him. Parents need to learn to control their kids, I'm a subscriber of the 'parent license' idea (needing to pass a etst to prove you can handle being a parent before you can have kids). So if some parent is too crappy to stop their kid from biting people (which really is dangerous anyway, what if the person really was infected with such a condition?) then a good scare like that is most certainly called for, I feel.
Fortunately, I've never had to deal with that, which is weird because I work in a supermarket part time and the worst I've had is kids screaming and crying. And even then, very little, it's actually much worse on public transport (which I tried to avoid anyway at university since I always lived within cycling distance of anywhere I needed to go, and the train station). Though my Nan has stories of me as a little kid when I apparently used to love winding up my late grandfather, particularly on holiday (apparently I would be playing somewhere and he'd call for me to go back to the Butlins/Haven/Whatever chalet, I'd wait for him at the top of the climbing frame, and as he slowly climbed the steps I'd wait, then when he got to the top go down the slide and run to the other side, and rinse and repeat - took him forever to catch me, usually when I got bored

).