Human history has been marked by significant migrations and adaptation to new methods of survival, with consequences for the infectious disease environment, health, longevity, and population. This chapter briefly outlines evolutionary changes in the human condition that led to agriculture, the growth of towns, and exposure to new infections, and considers the infectious disease environment in England when the long-term decline in mortality began. The infectious diseases that contributed to high mortality in the 18th century and the relationship of mortality decline to population growth and food supply are a particular focus. Population increased despite adverse changes in the infectious disease environment, and survival was dependent on immune defenses as well as food supplies. The importance of immunity may be less obvious now that infectious diseases in Western countries are relatively mild and appear to cause few deaths, although immune responses to invasive microorganisms occur continuously throughout life, whether or not there is any discernible acute sickness. In some cases the immune response can become less efficient with age, while other microorganisms, or the immune response to them, can have adverse biophysiological consequences resulting in chronic disease.

Although microorganisms causing some of the infectious diseases that were major killers in the 17th and 18th century are no longer prevalent in England, other common microorganisms continue to cause childhood epidemic diseases, respiratory infections, and gastrointestinal disorders. Some of these microorganisms are associated with chronic diseases that are now major causes of death. Although environmental, socioeconomic, lifestyle, and behaviorrelated risk factors contribute to the development of these diseases, common microorganisms have been confirmed as causal agents in some chronic diseases, while the involvement of infection in the underlying biophysiological processes in other chronic diseases is strongly suspected. Some chronic disorders have been linked with long-term effects of childhood infections or exposure to microorganisms during adult life, which will be considered in chapters 10–13 along with the issue of the criteria that can be applied to assess causality. At this stage, the possibility of infections being involved in chronic diseases can be borne in mind in considering evolutionary changes that have affected humans, their infectious disease environment, and their immune responses.