Had a science teacher ask me, "What is the vector for Spotted Fever?"
I knew that one, and replied, "The Rocky Mountain Tick."
Teacher said, "That is right!"

Then the teacher asked, "What is the vector for malaria?"
Again, I was feeling smug and answered, "The mosquito."
Teacher wooed me on by saying, "What a brilliant student! That is absolutely correct!"

Then the teacher, looking mischievous, asked, "What is the vector for AIDS?"
I was stumped, told the teacher, "I am afraid I don't know that one."

And smiling widely, the teacher replied, "Ass Hoppers!"