(from ebookjapan):
A strange custom that is still passed down in local villages. It is called "the milk frog". A frog would suckle instead of a baby who wouldn't drink milk, and suppress the mother's pain.... A mother went mad and slaughtered frogs one after another, and died a strange death. Yuki, the baby left behind, was transformed into a demon child who craved human blood instead of milk...!　Was it all the curse of the milk frogs?