The term military medicine has a number of potential connotations. It may mean a medical specialty, specifically a branch of occupational medicine attending to the medical risks and needs of soldiers, sailors and other service members. This disparate arena has historically involved the prevention and treatment of infectious diseases, and, in the 20th Century, the ergonomics and health effects of operating military-specific machines and equipment such as submarines, tanks, helicopters and airplanes. Undersea and aviation medicine can be understood as subspecialties of military medicine, or in any case originated as such. The planning and practice of the surgical management of mass battlefield casualties and the logistical and administrative considerations of establishing and operating combat support hospitals. This involves military medical hierarchies, especially the organization of structured medical command and administrative systems that interact with and support deployed combat units