I think you could might be able to catch something from drinking from a persons glass but funny when I was in High School I washed dishes in a fancy restaurant. Sometimes a New York Steak au Poivre, Veal etc would come back hardly touched and the servers would actually eat it.
I told this guy thats disgusting and he replied "Look here kid (hottie in the dining area), you'd be snogging her in the toilets if you had a chance, challenge your immune system".