There is this little boy about 12 years old walking down the sidewalk pulling a wagon and dragging a flattened frog on a string behind it, when he comes up to the doorstep of a house of ill repute.
He knocks on the door and the madam comes to answer it, sees him and asks what he wants. He says he wants what she is selling inside, and has the money to buy it and isn't leaving until he gets it. She thinks she could have some fun with him, so she tells him to come in.
Once he gets in, she tells him to pick one of the girls he likes. He asks her if any of the girls have any diseases and, of course, the madam says no. He tells her he has heard all the men in town talking about having to go to the hospital and get shots after making love with Mable, and she is the girl he wants and he has the money to pay for it.
The madam tells him to go upstairs and go to the first room on the right. So he heads down the hall dragging the frog behind him. Ten minutes later, he comes back down still dragging the frog.
He pays the madam, picks up his wagon and starts to head out the door. The madam stops him and asks him just why he picked the only girl she had in the place with a disease, instead of one of the others.
He replies, "Well, if you must know, tonight when I get home, my mother and father are going out to a restaurant to eat, leaving me at home with a baby-sitter. When they leave, I am going to make love to the baby-sitter and give her the disease I just caught. When mom and dad get home, dad will take the baby-sitter home and, on the way, he will make love to her and he will catch it. When dad gets home, he and mom will go to bed and they will make love and mom will catch it. In the morning when dad goes to work, the milkman will deliver the milk and he will make love to mom and he will catch it, and HE IS THE SON-OF-A-BITCH WHO RAN OVER MY FROG!!!!"