Idiopathic hypercalcaemia of infancy — the condition behind this entry’s “ideopathic” spelling — is a rare disorder in which newborns develop dangerously high blood calcium for no identifiable cause. Affected infants fail to thrive, vomit, become constipated and dehydrated, and may suffer kidney damage; severe cases were historically fatal. The mid-20th-century outbreaks were eventually traced to excessive vitamin D fortification of infant foods — a chastening lesson in the dangers of over-fortification — while truly idiopathic cases involve abnormal vitamin D sensitivity (now linked to CYP24A1 mutations). Treatment stops all vitamin D and calcium intake, hydrates aggressively, and uses steroids. It stands as a landmark warning: even vitamins poison in excess.