[Paul K]

A little paper bag was feeling unwell, so he took himself off to the doctors.
"Doctor, I don't feel too good," said the little paper bag.
"Hmm, you look OK to me," said the Doctor, "but I'll do a blood test
and see what that shows, come back and see me in a couple of days."
The little paper bag felt no better when he got back for the results.
"What's wrong with me?" asked the little paper bag.
"I'm afraid you are HIV positive!" said the doctor.
"No, I can't be - I'm just a little paper bag!" said the little paper bag.
"Have you been having unprotected sex?" asked the doctor.
"NO, I can't do things like that - I'm just a little paper bag!"
"Well have you been sharing needles with other intravenous drug users?"
asked the doctor.
"NO, I can't do things like that - I'm just a little paper bag!"
"Perhaps you've been abroad recently and required a jab or a blood transfusion?" queried the doctor.
"NO, I don't have a passport - I'm just a little paper bag!"
"Well", said the doctor, "are you in a homosexual relationship?"
"NO! I told you I can't do things like that, I'm just a little paper bag!"

"Then there can be only one explanation." said the doctor

Spoiler: 

"Your mother must have been a carrier"