I read this book back in middle school (about five years ago), and I can't seem to remember what it was. It wasn't too long. Maybe about 200 pages. The cover of the book was orange, because I recall the cover being a sunset on a canyon, two very orangey things.
It was about a girl, her grandfather and brother. They survived a disease that killed their parents and everyone else, except for those who were immune like them.
All I can remember is this group of bad people who were after this girl for some reason. She met a boy and they read some tale or myth about a fountain of youth or whatever. So they go looking for it and don't find it. So they end up walking into the sunset together.
I recall there being a light house and canyon, and the letters K and H keep popping into my head. I’m pretty sure the bad guys’ group name started with a K, and that the book's name, or the girl's name, started with an H. Or “the H...”