Hearteater

Hearteater, also known as "The White Death", is a disease that affects the lungs, despite its name. It doesn't kill many per year in humans, and normally those who die are the young, weak, and elderly. Adults usually spend a few days in a fever, and then, they recover slowly.

Spoilers from "Last of the Wilds"

Hearteater was unknown to the Siyee, as Si is too remote, however after the Circlian-Pentadrian War it was brought back and became a plague. This is due to the fact Siyee are smaller than humans, and less able to fight human diseases. It started in the North River Tribe, and was quickly spread to the other tribes, despite efforts to contain it. Hearteater began claiming Siyee lives of all ages. Tyve and Reet alerted Mirar of the disease, after meeting him in a cave in Si. Recognising the disease, he headed for the tribe and began to heal the ill. Auraya was also alerted and she too flew to help. Putting aside their issues, Mirar and Auraya worked together to heal the Siyee, isolating the ill, and containing the spread of the disease as much as possible. They continued for months saving lives, until Auraya was commanded to kill Mirar. After she refused and reliquished her position as White, she returned to Si to continue to heal the Siyee.