The way I see it, it'd be like the original setting for Bug City but with no bugs and everywhere. I'd go with this set up and give it a few twists to stop obvious ghoul busting ideas.
First, the slay ghoul spell--
One less Ghoul--does in fact exists (I personally have at least one character with it). So you just need a larger number of ghouls. Give a couple either the Spell Resistance or Arcane Arestor talents and that'll shore them up against such attacks.
As for a chemical agent designed against the ghouls--i.e. Strain III anyone?--you can have a few magician ghouls with the Purify spell (or whatever its called that stops Strain III) and that'll put an end to that.
Just have them attack a bunch of folk in a condensed area, most get away but bitten, and then multiple as it goes on. I wouldn't even start it out that way but instead have a few trid reports in the background which start to get more and more noticeable until its full on.
That's my take on it.
Sign--
Wacky