You can't get HIV/AIDS from smoking with people. Unless they slobber all over the joint/piece AND leave blood on it, there's no risk. Only if there's blood in the saliva will you be at risk.. and even then, the next person would have to have an open sore or something in their mouth. It's possible, but very rare.

Anyways, of course you can spread viruses and colds smokin' with other people. I've had a cold the last week or so and I let my friends know that I did. They still smoked w/ me. Even though I don't slobber and shit, I always made sure I wiped off the piece well after I took a hit. Don't want to get my friends sick!