Back in 2006 I did an "infected" gametype called Hitman for COD2. Basically, the gametype starts with 1 hitman, 1 commander, and the rest are guards who are supposed to protect the commander. As the hitman kills the guards, they turn into hitmen. Change the name "Hitman" to "infected" and you have the same thing: as the "infected" (= hitman) kills the guards, they become infected themselves. So, the gametype uses a method to increase the size of hitmen/infected exponentially. Which is what you were looking for.
Download Hitman gametype for COD2 here:
http://callofduty.filefront.com/file...Gametype;68302