Here's a good one:
"Q. Could a person be immune to the HIV virus, Doc? I have a very strange situation unfolding here. In 1993, I had unprotected sex with a young girl in the United States of America. Two years later, she died of complications from AIDS. I heard of this, and did an HIV test forthwith. And it came out negative. I met this other girl in 2010 and had unprotected sex with her around 100 times over a one-year period. After that, I met her sister, and had sex with her even more frequently. Two months ago, I learned that both sisters were HIV-positive. So I immediately did another test, as a matter of fact three tests over the last two months. And they all came in negative. What is going on here? Why do I not have HIV?"
A. [Mine, not the doctor's] It could be you are either very lucky or HIV resistant. However, one thing remains clear. You are a fucking idiot!