Delirium by Lauren Oliver

Lena can't wait for her Procedure. She is terrified of contracting deliria nervosa, what we know as love,  and will be immune once she has her Procedure. She's only got a few months to go when she meets Alex. Alex has the marks of the Procedure, so Lena isn't worried about the deliria with him. After all, he is Cured. But as Lena spends more time with him, she realizes that he knows a little too much about the Wilds that lay outside of the fence, talks about the Invalids without fear, and that she's in too deep with him to care.

Honestly, this story started a little slow for me. It had the feeling that it was giving a lot of background information, but I was anxious for it to really get started. Once it did, though, there was no putting it down! And the ending? Make sure you have Kleenex handy...