Biological warfare, “Public Health in Reverse,” calls for new methods of fighting disease, because when disease is willfully spread, it can take on new aspects. By understanding why an enemy may choose to use BW instead of some other weapon, we may be able to forecast its use and prepare to repel it. Various BW agents, means of distribution, and required properties are discussed. Although counteracting forces now exist in the health services of the United States, we must fashion and learn to use special defensive weapons. The author outlines four essential elements in a program of defense against BW.