By way of definition, biological warfare is the intentional use of living micro-organisms or their toxic products for the purpose of destroying or reducing the military effectiveness of man. Man may also be injured secondarily by damage to his food crops or domestic animals. The military objective, of course, is to reduce the will or capability to wage war. It has sometimes been said that BW is public health practice and procedure in reverse. This is an erroneous conception. BW is the deliberate use of natural disease agents whose inherent potential has been exploited by scientific research and development resulting in the production of BW weapons systems. Military campaigns and troop concentrations have always provided a fertile field for nal, urally occurring epidemic disease. Infectious disease has often been the critical factor in the outcome of a military campaign. Bubonic plague was said to have stopped the Crusaders at the very gates of Jerusalem. Dysentery probably caused more casualties in Napoleon's Grand Army than enemy firearms. Typhoid fever and dysentery played no favorites among the opposing forces in the Civil War, the Boer War, and the Spanish-American War. The new science of bacteriology was in its infancy around 1900. Even during World War I, infectious disease was a controlling factor in some campaigns. It is quite clear that typhus fever prevented the Germans from carrying through their Balkan campaigni. In spite of all our modern sanitation and preventive medicine, infectious disease contributed a large share to the cost and difficulty in World War II. Malaria ranked high as an enemy, both in the Mediterranean and in the Southwest Pacific, and scrub typhus caused some 7,000 casualties in the latter area. Anid finally, it seems almost yesterday that enteric infections, Japanese B encephalitis, and hemorrhagic fever were bedeviling us in the Korean conflict. In the past, a number of crude, unscientific, and purely local efforts were made to utilize infectious disease for military purposes. Alexander attempted such exploitation by catapulting the bodies of dead men and animals over the walls of besieged cities. It is reported that smallpox was started successfully among the American Indians during the French and Indian War by distributing blankets contaminated with purulent smallpox material. In World War I the Germans infected with glanders horses that were consigned from this country to the Roumanian cavalry. During World War II a number of units of the German Occupation Forces, particularly in Eastern Europe, were said to have been the target of local sabotage efforts with bacteriological agents. Dr. Fothergill, scientific adviser to the Chemical Corps, Fort Detrick, Md., delivered this paper at a joint meeting of the Nevada State Medical Association and the Reno Surgical Society, at Reno, August 1956.