The epidemiological transition describes the long-term changes in disease and mortality patterns in a population and were described by Omran in 1979 as having four stages:

1. Age of Pestilence and Famine – with high mortality and low life expectancy, diseases included smallpox, cholera and influenza.
2. Age of Receding pandemics – mortality declines and life expectancy increases due to improvements in living conditions from economic development.
3. Age of Degenerative and Man-made Diseases – mortality declines further with continued increase in life expectancy from vaccines and medical advances, however more deaths are from chronic disease associated with ageing and those associated with lifestyle such as diabetes and heart disease.

These occur at different stages in different countries depending on their socio-economic factors and medical advance available.

Stage 4 proposed by Rogers & Hackenberg called the Hybristic stage is present in the US and other industrialised countries; it continues with lifestyle diseases and also includes social pathologies such as homicide, suicide, along with a resurgence in communicable diseases due to poverty.

There have been many outbreaks and warnings of epidemics in recent years from diseases such as Swine flu (H1N1) and avian flu, however here in Ireland we normally escape a large-scale epidemic, thanks in part to being an island nation. Warnings from the Health Service Executive (HSE) published in the Irish Independent in November 2018 mention that swine flu could make a return this year.