Diseases that are contagious through transmission of saliva range anywhere from the common cold, which is most common, all the way to Hepatitis B, which is less common than most people would have you think.
There are literally hundreds of diseases and pathogens that can be subjected to the human body through basic person to person interaction, and you simply can't avoid them all. We had a Medic that picked up Turberculosis (Which is an Airborne Pathogen) from speaking with a girl that just fell down and broke her leg, so he didn't feel the need to wear a mask. It can happen just like that, so don't spend too much time concerning yourself with it. Most of what you will pick up through sharing pipes or joints is treatable and/or curable through medication.
Leaving out the common cold and influenza viruses, here are some of the more serious conditions:
Upper Respiratory Infection
Mononucleosis
Meningitis
Bacterial Meningitis
Eppstein-Barr Virus
Cytomegalovirus
Polio
Hep B
Molluscum contagiosum
Cool, huh?
Just relax, Amigo. The chances of your friends having any of these are pretty slim unless you are really certain that they're living a double life.