Didn't the Cleric make some comment on it not infecting like it'd normally do so?
I can't be bothered to find the exact quote, but Swen did say that they created a special tadpole for the purpose of the game. One of the things mentioned was time limit, but the tadpole also grants host powers (enhanced physical abilities, mindmeld between different hosts). Accepting/rejecting powers coming from the tadpole is to be one of the central themes (a bit like accepting/rejecting heritage of Bhaal), and as it is to be somewhat tied to the original, I think it is safe to assume that there is more to it, then just fighting against invaders.
