D.J. MacHale 
Black Water [EPUB ebook] 

Breaking the rules Just when fifteen-year-old Bobby Pendragon thinks he understands his purpose as a Traveler — to protect the territories of Halla from the evil Saint Dane — he is faced with an impossible choice. The inhabitants of Eelong are in danger of being wiped out by a mysterious plague. The only way Bobby can stop it is to bring the antidote from another territory. Since moving items between territories is forbidden by the Traveler rules, if Bobby chooses to save Eelong he could endanger himself, his friends, and the future of every other being in Halla.