I think the premise was that those who had cancer took the specially formulated vaccine to cure their own existing cancer, but the vaccine utilized a type of virus that ended up spreading to everyone, wiping out the population quickly, except for a few who lived but mutated into a kind of monstrous albino cannibal creature that sleeps during the day and comes out at night to search for something to eat, as food was no longer being produced. (Haven't seen it since it came out, but that's what I remember.)