Smallpox was largely a disease of close contacts [17, 26, 33] , spread primarily through face to face contact with an infected person (or occasionally through contaminated clothing). Individuals in the incubation period of smallpox were not infectious, and long term carriers did not exist. Patients were believed to be infectious following the development of oropharyngeal lesions, which could precede the rash by 24 hours [26] . However, patients were believed to be most infectious during the first week of the rash [26] ; Dixon (1962) believed that patients could be infectious from the onset of acute viremia, but most evidence suggested that little transmission occurred prior to the development of the rash [26, 33] . The more severe the case, the more infectious they appeared to be [34] ; mild cases were believed to have very little infectiousness. While scabs contained infectious material and patients were considered to be infectious until the last scab fell off, in practice patients were not highly infectious during the scabbing phase. Importantly, patients who had been vaccinated were found to cause fewer secondary cases [34] . Very severe cases, such as hemorrhagic or flat smallpox, occasionally resulted in considerable transmission, owing to diagnostic difficulties; mild cases, in which the patient remained ambulant during the course of the disease, could cause considerable spread as well [35, 36] . Within a household or family dwelling, the secondary attack rate of unvaccinated susceptibles depended on the time and place, occasionally below 50% [29] , but often approaching 100% [37] . Drier conditions were often believed to favor transmission [17, 27] , so that lower rates of transmission derived from tropical regions may not be applicable to the temperate zone [38] . The number of secondary cases resulting from a given importation into Europe varied widely [39] , with most importations yielding few cases, but with the occasional large outbreak being seen.