Under a biological weapons attack, high percentages of the targeted population will become ill, and there will be high fatality rates. In a biowarfare attack, the usual route of infection will be by inhalation of an aerosol. They are invisible, tasteless, and have no odor. Victims will be exposed to an invisible cloud. Strange epidemics with unusual organisms should be suspect. Localized epidemics of disease related to wind direction may occur. Bacterial, toxin, and chemical weapons systems are the most likely to be used in such an attack. As a defense, protective masks could be used to exclude bacteria from entry. There must be some warning, however, to put on a mask. If there is an attack, other concentrations of troops should be put on alert. These units could then begin to wear light-weight masks so as to undermine the enemy's confidence in another successful attack. Terrorists may not have used such weapons thus far because of the risks of retaliation such a strategy holds. They also may not yet have effective biological weapons.