I'm looking for a certain book. I definitely read this prior to 2004, but I can't narrow it down any more than that (sorry).
It was a sci-fi book about what I think was an alien invasion of Earth, but it could also just have been a parasite. The main character was a teenage (I think) female, who was exploring some sort of (maybe) underground...factory? Cave? when one of her friends accidentally cuts himself on some scrap metal or other sharp object. Turns out that's how you get infected by these alien parasites, and they slowly start taking over everyone the main character knows. The only way to know if someone is an alien or not is that they have black blood.
The only thing I can distinctly remember is that at the end of the book, the MC is trying to run away in a car with her boyfriend and little sister. The boyfriend says that the little sister might be an alien; she's sitting in his lap and the BF says that since the little sister's wearing a white undershirt it'd be easy to prick her with a needle and see if she bleeds. The MC agrees and it seems that the little sister is also an alien.
I can't remember what happens to the little sister, but eventually the MC and her BF are in a motel room and she figures out that the BF was tricking her - he's actually the alien. She makes to attack him with a fork. I'd like to figure out what this book is so I can find out what happened in the end for once and for all.