The Dragon Death Plague
Roughly one-hundred years ago the human race was decimated by a plague that was to become known as the Dragon Death.
The name was due to the effects of the plague, which caused the skin to become inflamed and scaley, and appear very much like the scales of a red dragon.
Only a few humans survived, but the human societies collapsed under the rapid spread of death.
The plague left a huge power vacuum, that was gradually filled by other intelligent races.