Since I can't figure out exactly how to describe Savannah Grey by British author Cliff McNish, I'm going to use the summary on its jacket flap:

Savannah Grey needs to keep moving. She doesn't know why, but she can't let herself get tied down by too many people. It's almost like she's being chased by something. And now something strange is happening with her neck - with her throat.

Savannah Grey never thought she'd meet someone like Reece - a guy who seems to understand her. He even knows about her neck. The same thing is happening to him. It's as if their voices are becoming weapons, warming up for some kind of attack.

Savannah Grey has no idea what might be chasing her or why her voice suddenly feels like the most powerful weapon on the planet, but she's about to find out.

Nature is preparing for battle with the universe's ultimate monster. The time to fight is almost here.

The weapon is Savannah Grey.