SHaring cigarettes please help

So me and a friend were smoking and he gave me a puff of his cigarette. I burned the filter to kill any of his bacteria just in case but was still anxious. Next the dirty f***er said he found it in an ash tray. I’m so scared I’m gonna get a diesease now 

Just breathe! You'll be ok... Especially if you burnt the end. If it helps... I used to be homeless and would always smoke butts from the ground and dirty ash trays. Worst thing you might get would be mono or something treatable.

Definitely don't be scared about getting a disease from the cigarette that was in an ash tray, especially if it's from someone you know. It's okay. The only thing that would effect you I'd the cigarette itself... I'm just saying... I smoked for awhile, but I was able to quit. I hope you settle your mind.