Okay, here’s a good one that everyone seems to ignore:
Marijuana has many health benefits, but keeping a person from getting a cold isn’t one of them. If you’re sick, make sure to tell everyone. Better yet, bring your own one-hitter or cup your hand around the mouthpiece to avoid cross contamination.
It’s from the excellent Colorado Pot Guide.
I don’t know why everyone forgets about this. Even stoners who are excessively germ-conscious in other contexts — like sharing drinks — often have no compunctions about sharing a slobbery blunt. I have been told that the heat from smoking kills bacteria, which is undoubtedly true to some degree but not enough to sterilize it, certainly (maybe a cheapie one-hitter, since they get very hot indeed, but not a bowl or bong, and certainly not a joint or blunt).