It's aging, the one thing that all people succumb to if they don't die of something else first. In the story, the dragon is widely believed to be both inevitable and a necessary part of life, but there are always a few who thought otherwise. It was only when science reached a certain point did it become possible to see how the dragon might be slayed, potentially saving the increasing numbers of people taken by it every year as the human population grows.

Aging is the only thing that matches the dragon.