A recent population-based cohort study published in The Lancet explored variations in common diseases, related hospital admissions and mortality across individuals aged 35-70 years from 21 countries across five continents. What was particularly unique about this study was that the observations were studied and documented by country type i.e. whether it was a high-income, middle-income, or low-income country. An earlier study published in the Annals of Internal Medicine followed a similar strategy but was limited to the United States. The results from both studies indicate that there is an epidemiologic transition occurring leading to heart disease being surpassed by cancer as the leading cause of death in high-income communities.

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An epidemiologic transition is characterized by changing patterns of age- and cause-specific mortality and was originally proposed by Abdel Omran in 1971. This model was revised and expanded in subsequent years and now includes four phases of transition:

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1) The Age of Pestilence and Famine: This phase is characterized by epidemics of infectious diseases, high mortality with wide swings in mortality rate, low life expectancy between 20 to 40 years, and low population growth.

2) The Age of Receding Pandemics: In the second phase, the incidence of infectious diseases declines as does the mortality rate, non-communicable diseases emerge, population growth is sustained and life expectancy increases to 30 to 50 years.

3) The Age of Degenerative and Man-Made Diseases: Phase 3 sees a continuing rise in life expectancy to beyond 50 years and a changing pattern of cause of death that is now largely related to chronic diseases such as cardiovascular disease, cancer, and degenerative conditions, and man-made causes such as violence, accidents and substance abuse.

4) Age of Delayed Degenerative Diseases: Phase 4 was added to Omran’s original theory to incorporate advances in medicine and technology that led to a stabilizing effect on mortality and fertility, and lifespan being extended beyond 70 years. New diseases are emerging due to antibiotic resistance, new pathogens and mutations, but degenerative diseases are largely controlled for those with access.

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The epidemiologic transition theory has its supporters as well as critics but there is no doubt that the world has been seeing changes in patterns of fertility, mortality, life expectancy and leading causes of death and morbidity over time. Understanding these patterns is extremely important in ensuring effective public health planning at local, state, national and global levels. While there are variations across countries and due to socioeconomic, ethnic and gender inequalities, even those patterns tell a story and there are lessons to be learned that can be applied within our respective local context.