" A Canadian physiologist named Norman Kasting discovered that bleeding animals induces the release of the hormone vasopressin; this reduces their fevers and spurs their immune system into higher gear. The connection isn’t unequivocally proven in humans, but there is much correlation between bloodletting and fever reduction in the historic record. Bleeding also may have helped to fight infection by reducing the amount of iron available to feed an invader, providing an assist to the body’s natural tendency to hide iron when it recognizes an infection. "

― Sharon Moalem , Survival of the Sickest: A Medical Maverick Discovers Why We Need Disease