Infectious disease used to be the major brake on population growth. Nobody was free of risk, but children and women in childbirth were especially vulnerable. This dismal situation improved in industrial societies of the nineteenth century because of a host of social advances, including improvements in sanitation, nutrition, and housing and better understanding of hygienic principles. Although the potential of vaccination was known in the early nineteenth century, mass immunisation and antibiotics contributed importantly only after 1940. We now have weapons of enormous power that drastically reduce the incidence of infection and terminate ongoing infections, such as blood poisoning and pneumonia, that formerly took so many lives. However, the public health officials of the 1960s who complacently rejoiced in the success of preventative medicine would have been surprised to know how frequently new diseases would emerge in the subsequent forty years.
Nobody knows exactly when infectious disease became a great threat to human existence, but the first settled communities were probably much more vulnerable to devastating epidemics than their nomadic ancestors. Demographers believe that although the birth rate was high, these communities grew slowly and spasmodically because of periodic outbreaks of disease. Without efficient sanitation, sound hygienic practices, or immunity to these diseases, human societies were always at the mercy of microbes, a susceptibility greatly increased by malnutrition. The Old Testament is, again, an indication of the perceptions of an ancient people (see Fig. 9.1).