Smallpox was a lethal infectious disease and was persistent in England throughout the period 1600–1800; perhaps 20% of smallpox victims died (although a very much higher mortality in London in the 18th century is suggested by the modelling in Chapter 12) and the survivors retained an immunity for many years. Evidence suggests that a particularly virulent strain began to afflict people of all ages and social backgrounds in the middle and later decades of the 17th century.

When smallpox was fully established in large conurbations (with high N/β) it was endemic and was characterised by 2-yearly epidemics, the evolution of which, from 4-yearly through 3-yearly epidemics, can be traced clearly in London (section 9.5) and Chester (section 10.2). It has been suggested that smallpox was also endemic in such cities as Nottingham, Chester, Northampton, Norwich, and Manchester after 1725 and before it was brought under control by inoculation and then by vaccination (Mercer, 1985). There are no long runs of data for deaths or cases of smallpox in rural England outside the major cities during the 17th and 18th centuries, although there are sporadic references to smallpox deaths in many parish registers and many accounts of outbreaks of smallpox in rural England in the 18th century (Creighton, 1894; Razzell, 1977), but this does not necessarily mean that the disease was endemic; indeed, accounts of spectacular outbreaks with widespread mortality indicate a population that had not recently experienced the disease and in which most individuals were susceptible. An example is the outbreak in the Island of Foula (Shetland Islands) in 1720, where only about six persons survived out of a population estimated at 200 (Razzell, 1977).