The loss of appetite that typically accompanies infection or the mere exposure to parasites is traditionally considered a negative by-product of infection, benefitting neither the host nor the parasite. In fact, numerous medical and veterinary practices directly or indirectly subvert this ‘illness-mediated anorexia’. However, the ecological factors that influence it, its effects on disease outcomes, or why it evolved remains poorly resolved. We explore how hosts use anorexia to defend against infection and how parasites can manipulate anorexia to enhance transmission. Then, we use a coevolutionary model to illustrate how changes to anorexia could alter disease dynamics and virulence evolution. Anorexia could be exploited to improve host health and disease management; we propose an interdisciplinary approach to minimize unintended consequences.