So, I assume sudden slimate change caused by the impact resulted in the collapse of society? You must realise, though... that big pieces of space debri (especially if you're talking about small planetoids, astroids would be better) might cause the destruction of life around the earth. It happened 65 million years ago, it could happen again. So, don't make it too big.
Or, if it's not the impact that caused the landing of the desease on earth, you have the explain other ways for it to land in the middle of our solar system.
A non-cliche antagonist? You have my interest. Got an idea about how you're going to do that? A group works just as fine as a single individual. As long as each character is interesting in their own way, that is. :3