Black Iron Sickness (黒鉄病, Kurogane Byou) is a disease thought to be caused by the awakening of the Tree of Zetsuen. It is actually caused by the Tree of Beginnings trying to stop the Tree of Zetsuen from awakening. It only happens in areas where the Fruit of Zetsuen emerge from the ground which is usually accompanied by swarms of butterflies. It causes all animals to metallicize, exept the butterflies that follow the Fruits of Zetsuen.
The Black Iron Sickness can not affect those who can use magic or have a magical item, nor the people near them.
Mahiro was said to save a girl from another village from the Black Iron Sickness because apparently she looked a lot like Aika; Yoshino on the other hand tried to warn a couple about their impending doom with the iron sickness but they ignored him. Afterwards he tried to save two boys who were playing on the playground, by throwing a talisman ring towards them, but he was too late, as the two boys turned into metal.