I was just killing time on some french forum, then I saw a thread whose title was "Weird disease" and someone linked this wikipedia page.

"Witzelsucht is a set of rare neurological symptoms characterized by a tendency to make puns, or tell inappropriate jokes or pointless stories in socially inappropriate situations."

We then read :

"A less common symptom is hypersexuality, the tendency to make sexual comments at inappropriate times or situations."

Alright alright, but what's the link with the Bite of 87, you may ask ? Well, this appears a few lines later :

"This disorder is most commonly seen in patients with frontal lobe damage, particularly right frontal lobe tumors or trauma."

When I read that line, everything went dark, I couldn't breathe, because I knew...

I knew I had just discovered the truth about the Bite of 87.

Sexual jokes ? Frontal lobe ? Here's what Phone Dude says on Night 1 :

"y'know, to make sure no one steals anything or makes out in the corner, but you'll also be a part of the show."

This joke, Phone Dude's happy temperament... I got it, guys, I just solved FNAF... Phone Dude is the victim of the Bite of 87...