I avoid sharing drinks and whatnot because I apparently got some seriously bad cold sores from sharing a beverage. I think that the vast majority of the population has the herpes cold sore virus, so if you're concerned you shouldn't share anything with anybody, although I'm guessing that sharing a pipe is less risky then sharing a drink. You could always sanitize the pipe with alcohol before using it, or flame it with a lighter. You can also get the bacteria that causes ulcers, and all kinds of other stuff, from sharing pipes and beverages and the like.