Shared a joint with a homeless guy, could i have caught anything?

i realise this probably wasnt my brightest idea. i gave 10 bucks to a homeless guy today, and he was smoking a joint. he asked me if i wanted any, so i obliged and we chatted and smoked for afew minutes sharing it back and forth. i realise now that i have no idea where his mouth has been. is it possible to catch anything from saliva on rolling paper?