Changes in Disease, Mortality, and Life Expectancy

Human beings have been adapting to invasive microorganisms contracted in different environments throughout their history, and long before the existence of microbes was known. The recent epidemiological transition has occurred very late in human history, along with urbanization and industrialization in the last three hundred years. Earlier changes in the predominant way of life exposed humans to new infectious diseases that challenged the immune system and people's ingenuity as they sought to avoid or prevent them. A transition in disease patterns occurred when microorganisms transferred to humans as a result of following herds of animals for food. Later, during the long period of the “Neolithic Revolution,” the development of crop cultivation enabled people to live in more settled communities alongside domesticated animals, thus increasing exposure to new infections. Living in villages and eventually towns led to infection with airborne, waterborne, and food-borne microorganisms that were able to adapt to humans as suitable hosts. Human settlement also resulted in new microbial hazards due to living in close proximity to animal and human waste, which could be more easily avoided in a nomadic way of life. Fecal contamination of water and food led to new waterborne and gastrointestinal infections. The increase in size of centers of population brought further changes in the infectious disease environment, with airborne viruses such as measles and smallpox causing regular epidemics among those with no immunity, or becoming endemic where populations were sufficiently large and concentrated. Infectious diseases spread as a result of migration from areas of high prevalence, via rural-urban migration, as a result of increasing contact between people in rural areas and towns, and through trade. Human beings had evolved for a very different way of life of hunting and gathering, and adaptation to a “modern” way of life is still far from complete.

This study of changes in disease and mortality in England since the 17th century indicates the continuing importance of the infectious disease environment despite the dramatic decline in death rates from acute infectious diseases and tuberculosis, as many of the chronic diseases that have replaced them as the predominant causes of death have confirmed or suspected infectious etiology.