The Plagues of Kondar by Lynne Kositsky

Arien holds the key to healing her planet.

Planet Kondar has a light side that faces the sun and a dark side in eternal night. Lightsiders have never met those on Darkside, known as Oscura. Arien lives in Kattannya on Lightside. When her parents fall through thin ice and drown, she is sold in the marketplace. The chief seer of Vor, Yaddair, purchases her.

Vor is very close to Edge, a grey wall of fog that divides Lightside from Oscura. The Oscurans are suffering from a deadly plague and some fly into Vor, bringing the disease with them. How will the Vorians cure it? And what will happen to Arien?