Actually, there are a number of terrestrial parasite species which have ridiculously convoluted life cycles involving lots of very specific and unlikely circumstances.
And many of those do something that compels the host to help the parasite complete it's life cycle, like the fungus that kill insects or the one worm that infects grasshoppers. Good point. The Slither life cycle is insanely complicated, even compared to those.
While I think that the 2nd half pales somewhat to the 1st half, the movie is still a great time to be had. It kind of reminded me of Night of the Creeps in terms of the plot at times but it still manages to be it's own entity.
Oh yeah, and Elizabeth Banks is one gorgeous woman!
Banks doesn't really do it for me as a sex symbol, but I think she was very good in her part. I will speak up for the movie's second half, because it includes the fiendishly out of place use of "Every Woman in the World" and also several funny lines. My favorite is the one exchange between Bill and Kylie.
Maybe a 50" big screen TV would have solved the Grants' sexual problems. According to one poll taken in the U.K. 50% of all British men would give up sex for a 50" big screen TV. It couldn't hurt.
Ah, but Grant Grant is a sugar daddy. He already has the big TV.