>>17
Epidemic mutagenic disease - Like AIDS, only it can change DNA as well as RNA. Sexually transmitted, of course. The Goblin girls are what's left of the Halflings. The elves fled to Arvandor. The Dwarves may still be living underground, but nobody ever sees them - they're in full isolation mode. The Ogres and ogre-kin became the Red Oni. The Kobolds became the Lizardmen.