In most stories, the antagonist is a human character. But that's not always the case, as we can see in Shiver by Maggie Stiefvater. In this book, the antagonist is not a human being but rather the wolf virus, which is gradually turning Sam into a wolf.
Sam doesn't want to become a wolf; he'd much rather retain his humanity. But that's easier said than done, and Sam's girlfriend Grace is at a loss to know how to avoid losing him to the pack. She's desperate for something that will cure Sam of being a wolf.
Grace is joined in her quest by Isabel, the sister of Jack Culpeper. Jack was killed by a pack of wolves only to come back to life as a wolf himself. Understandably, Isabel wants her brother back, but as a human, not as a wolf, so she faces the same antagonist as Grace—the wolf virus.
The quest for a cure brings out certain antagonistic traits in Jack, who traps Sam and kidnaps Grace in an attempt to find out why Grace, who was once bitten by a wolf, hasn't turned into one herself. As it turns out, Grace contracted a fever not long after she was bitten, and that's what prevented her from turning into a wolf.
With this information, Isabel sets out to defeat her and Grace's antagonist by injecting Sam, Jack, and Olivia (one of Grace's friends) with a blood sample from a man with meningitis. She hopes that doing this will induce a powerful fever capable of killing the wolf virus.