Because natural infection was thought to occur via large droplets deposited on the upper respiratory mucosal, the success of nasal inoculation in producing low mortality rates has been hard to understand. A theory suggested by Henderson to the author of a smallpox history, (Hopkins, 1983, p. 114 ) "is that virus inhaled naturally was in sufficiently small particles to be deposited deep within the lung, whereas particles inoculated by nasal insufflation may have been much larger and were likely to implant in the nose or throat where [only] a local lesion might be produced." The relative importance of age and health of inoculated subjects, infectious dose, and route of exposure are not known.