I already posted in another thread, but I'll throw it up here.
Little Johnny goes to a brothel and tells the girl at the counter "I want the nastiest girl you have, preferably with AIDS." The lady looks at him confused, then points him to a room. Little Johnny goes to the room and bumps uglies with the prostitute. After they are done and they're getting dressed, the hooker asks him "So, just to satisfy my curiosity, why did you want to get AIDS?" Little Johnny replies:
"Ok, I'm gonna go home and **** my babysitter, then my babysitter will have AIDS.
"Then my babysitter is gonna **** my dad, and my dad will get AIDS.
"Then my dad is gonna **** my mom, and my mom will get AIDS.
"Then my mom is gonna **** the mailman, then the mailman will have AIDS.
"And that's the mother ****** who ran over my turtle."
I have heard Lemmy Kilmister tell this joke on several occasions. still gets me laughing.