It's a genetic abnormality which resulted when you went back in time and performed certain actions which made you your own grandfather.
Fry: "I did do the nasty in the pasty!"
Nibblonian 2: "Verily. And that past nastification is what shields you from the brains. You are the last hope of the universe."
Fry: "So, I really am important? How I feel when I'm drunk is correct?"
Nibblonian 1: "Yes. Except the Dave Matthews Band doesn't rock."
