The Call by Lisa Shapiro

Humans are always pushing the boundaries of space travel. When they discovered the planet Ganelle, home to a peace-loving humanoid race adapted to life on land and in water, it seems too good to be true. It is when a virulent disease spreads and drives its victims to madness and horrific death. In The Call, the humans must go to Kel, an outlawed group from a distant colony that knows the secret link between humans and Ganellians. By joining forces with Kel, the humans soon open themselves up to other dangers.