Experimental aerosol data suggested that poxvirus, which survived the trauma of artificial aerosolization, remained infectious for significant periods of time. Aerosols of vaccinia demonstrated a half-life of about 6 h at 22 • C and relative humidity ≤50% with reduced stability at higher relative humidity and temperature (Harper, 1961) . Variola appeared to have a similar half-life and not to be affected by relative humidity at 26.67 • C (Mayhew and Hahon, 1970) . Other experiments demonstrated that airborne vaccinia is highly sensitive to inactivation by germicidal ultraviolet light (Edward et al., 1943; Jensen, 1964) . Westwood et al. (1966) demonstrated that inhalation of a single PFU of a submicrometer vaccinia aerosol was sufficient to infect rabbits. Airborne rabbit pox was similarly infectious. They demonstrated rabbit-to-rabbit airborne transmission of rabbit pox in each of seven trials by placing uninfected rabbits in separate cages in the same room with infected animals. They also infected rhesus monkeys using submicrometer aerosols of variola.