I was told this when I was 7, by a uni student. I still love it.
It is just another working day at the brothel. The punters are coming, the cash is coming, and the girls are faking it. Suddenly the door opens, and a figure steps through the threshold, silhouetted against the backdrop of the red lights.

It is a boy, a young boy, about nine. In his hand is a length of string, reaching all the way to the floor, where it is fastened around the neck of what looks like a dead frog. The figure picks the frog up, strides up to the counter, and places the amphibian onto it, exclaiming, “I want a bitch with herpes.”

Behind the counter, the sexmonger is flabbergasted. “I’m sorry, what did you say?”

“I said ‘I want a bitch with herpes’” says the child.

“Well I’m afraid we don’t have any ladies that fit that description” replies the Madam.

“I want a bitch with herpes, and I am going to have a bitch with herpes” says the boy, putting £1000 on the counter, next to the frog.

The Madam thinks about this for a while and decides that the reputation of her house of ill-repute is definitely worth £1000. She sends for one of her many shingle infected sluts. The boy follows the genitally deformed working girl to her room, trailing his frog behind him.

After an hour of quite bizarre shenanigans the kid returns to the counter, frog in tow, to thank the Madam. She takes this opportunity to ask the question that had been puzzling her for the entire previous hour, “Why did you want a whore with herpes?”

“Well,” explains the boy “My parents are going out tonight. In the evening I’m going to fuck my babysitter. When my dad drives her back home, he’s going to fuck her. Then he’s going home to fuck Mum. In the morning she’s going to fuck the milkman. He’s going to fuck his wife, she’s going to fuck her boss, he’s going to fuck his wife - my English teacher, she’s going to fuck my headmaster and HE’S THE CUNT WHO KILLED MY FROG!”
(Nossidge, Thu 8 Dec 2005, 1:49, Reply)