In the most austere combat conditions, Yugoslav guerillas of World War II (WWII) demonstrated an innovative and effective hospitalization system that saved countless lives. Yugoslav Partisans faced extreme medical and logistical challenges that spurred innovation while waging a guerrilla war against the Nazis. Partisans used concealed hospitals ranging between 25 to 215 beds throughout the country with wards that were often subterranean. Concealment and secrecy prevented discovery of many wards, which prototypically contained two bunk levels and held 30 patients in a 3.5 × 10.5-meter space that included storage and ventilation. Backup storage and treatment facilities provided critical redundancy. Intra-theater evacuation relied on pack animals and litter bearers while partisans relied on Allied fixed wing aircraft for inter-theater evacuation.