Warm-blooded animals have elaborate physiological controls to maintain constant body temperature (in humans,37℃).Why then during sickness should temperature rise, apparently increasing stress on the infected organism? It has long been known that the level of serum iron in animals falls during infection. Garibaldi first suggested a relationship between fever and iron. He found that microbial synthesis of siderophores—substances that bind iron—in bacteria of the genus Salmonella declined at environmental temperatures above 37℃ and stopped at 40.3℃.Thus, fever would make it more difficult for an infecting bacterium to acquire iron and thus to multiply.