Epidemiological transition
In demography and medical geography, epidemiological transition is a theory which "describes changing population patterns in terms of fertility, life expectancy, mortality, and leading causes of death." For example, a phase of development marked by a sudden increase in population growth rates brought by improved food security and innovations in public health and medicine, can be followed by a re-leveling of population growth due to subsequent declines in fertility rates. Such a transition can account for the replacement of infectious diseases by chronic diseases over time due to increased life span as a result of improved health  care and disease prevention. This theory was originally posited by Abdel Omran in 1971. Theory. Omran divided the epidemiological transition of mortality into three phases, in the last of which chronic diseases replace infection as the primary cause of death. These phases are: In 1998 Barrett et al. proposed two additional phases in which cardiovascular diseases diminish as a cause of mortality due to changes in culture, lifestyle and diet, and diseases associated with aging increase in prevalence. In the final phase, disease is largely controlled for those with access to education and health care, but inequalities persist. The epidemiological  transition occurs when a country undergoes the process of transitioning from developing nation to developed nation status. The developments of modern healthcare and medicine, such as antibiotics, drastically reduce infant mortality rates and extend average life expectancy which, coupled with subsequent declines in fertility rates, reflects a transition to chronic and degenerative diseases as more important causes of death. The theory of epidemiological transition uses patterns of health and disease as well as their forms of demographic, economical and sociological determinants and outcomes. History. In general human history, Omran's first phase occurs when human population sustains cyclic, low-growth , and mostly linear, up-and-down patterns associated with wars, famine, epidemic outbreaks, as well as small golden ages, and localized periods of "prosperity". In early pre-agricultural history, infant mortality rates were high and average life expectancy low. Today, life expectancy in developing countries remains relatively low, as in many Sub-Saharan African nations where it typically doesn't exceed 60 years of age. The second phase involves improved nutrition as a result of stable food production along with advances in medicine and the development of health care systems. Mortality in Western Europe and North America was halved during  the 19th century due to closed sewage systems and clean water provided by public utilities, with a particular benefit for children of both sexes and to females in the adolescent and reproductive age periods, probably because the susceptibility of these groups to infectious and deficiency diseases is relatively high. An overall reduction in malnutrition enabled populations to better resist infectious disease. Treatment breakthroughs of importance included the initiation of vaccination during the early nineteenth century, and the discovery of penicillin in the mid 20th century, which led respectively to a widespread and dramatic decline in death rates from previously serious  diseases such as smallpox and sepsis. Population growth rates surged in the 1950s, 1960's and 1970's to 1.8% per year and higher, with the world gaining 2 billion people between 1950 and the 1980s. A decline in mortality without a corresponding decline in fertility leads to a population pyramid assuming the shape of a bullet or a barrel, as young and middle-age groups comprise equivalent percentages of the population. Omran's third phase occurs when human birth rates drastically decline from highly positive replacement rates to stable replacement numbers. In several European nations replacement rates have even  become negative. This transition generally represents the net effect of individual choices on family size and the ability to implement those choices. Omran gives three possible factors tending to encourage reduced fertility rates: Impact on fertility. Improvements in female and childhood survival that occur with the shift in health and disease patterns discussed above have distinct and seemingly contradictory effects on fertility. While better health and greater longevity enjoyed by females of reproductive age tend to enhance fertility, the reduced risks to infants and young children that occurs in the later stages of the transition tends to have the opposite  effect: prolonged breastfeeding associated with reduced mortality among infants and toddlers, together with parental recognition of improved childhood survival, tend to lengthen birth intervals and depress overall reproductive rates. Economic impact. The transition may also be associated with demographic movements to urban areas, and a shift from agriculture and labor-based production output to technological and service-sector-based economies. This shift in demographic and disease profiles is currently under way in most developing nations, however every country is unique and transition speed is based on numerous geographical and sociopolitical factors. Whether the transition is due to socioeconomic improvements (as  in developed countries) or by modern public health programs (as has been the case in many developing countries), the lowering of mortality and of infectious disease tends to increase economic productivity through better functioning of adult members of the labor force and through an increase in the proportion of children who survive and mature into productive members of society. Models of transition. Omran developed three models to explain the epidemiological transition. Other perspectives. McMichael, Preston, and Murray offer a more nuanced view of the epidemiological transition, highlighting macro trends and emphasizing that there is a change from infectious to non -communicable disease, but arguing that it happens differently in different contexts. One of the first to refine the idea of the epidemiological transition was Preston, who in 1976 proposed the first comprehensive statistical model relating mortality and cause-specific mortality. Preston used life tables from 43 national populations, including both developed countries such as United States and England and developing countries such as Chile, Colombia, Costa Rica, Guatemala, México, Panama, Taiwan, Trinidad and Tobago, and Venezuela. He used multiple linear regression to analyze the cause-specific-age-standardized death rates by sex. The estimated slopes represented the proportional contribution of  each cause to a unit change in the total mortality rate. With the exception of neoplasms in both sexes and cardiovascular disease in males, all of the estimated slopes were positive and statistically significant. This demonstrated that the mortality rates from each specific cause were expected to decline as total mortality declined. The major causes accounting for the decline were all infectious and parasitic diseases. McMichael et al. argue (2004) that the epidemiological transition has not taken place homogeneously in all countries. Countries have varied in the speed with which they go through the transition as well as what stage  of the transition they are in. The global burden of disease website provides visual comparisons of the disease burdens of countries and the changes over time. The epidemiological transition correlates with changes in life expectancy. Worldwide, mortality rates have decreased as both technological and medical advancements have led to a tremendous decrease in infectious diseases. With fewer people dying from infectious diseases, there is a rising prevalence of chronic and/or degenerative diseases in the older surviving population. McMichael et al. describe life expectancy trends as grouped into three categories, as suggested by Casselli et al.: Murray and Lopez (1996) offered  one of the most important cause-of-death models as part of the 1990 Global Burden of Disease Study. Their "cause of death" patterns sought to describe the fraction of deaths attributed to a set of mutually exclusive and collectively exhaustive causes. They divided diseases into three cause groups and made several important observations: The regression approach underlying the Global Burden of Disease received some critique in light of real-world violations of the model's "mutually exclusive and collectively exhaustive" cause attribution. Building on the existing body of evidence, Salomon and Murray (2002), further add nuances to the traditional  theory of epidemiological transition by disintegrating it based on disease categories and different age-sex groups, positing that the epidemiological transition entails a real transition in the cause composition of age-specific mortality, as opposed to just a transition in the age structure. Using Global Burden of Disease data from 1990, they disintegrate the transition across three cause groups: communicable diseases, non-communicable diseases and injuries, seeking to explain the variation in all-cause mortality as a function of cause-specific mortality in 58 countries from 1950 to 1998. This analysis validates the underlying premise of the classic epidemiological transition  theory: as total mortality declines and income rises, communicable diseases cause less and less mortality compared to non-communicable diseases and injuries. Decomposing this overall impact by age-sex groups, they find that for males, when overall mortality decreases, the importance of non-communicable diseases (NCDs) increases relative to the other causes with an age-specific impact on the role of injuries, whereas for women, both NCDs and injuries gain a more significant share with mortality decreases. For children over one year, they find that there is a gradual transition from communicable to non-communicable diseases, with injuries remaining significant  in males. For young adults, the epidemiological transition is particularly different: for males, there is a shift from injuries to NCDs in lower income settings, and the opposite in higher-income settings; for females, rising income also signifies a shift from NCDs to injuries, but the role of injuries becomes more significant over time compared to males. Finally, for both males and females over 50, there is no epidemiological transition impact on the cause composition of mortality. Current evidence. The majority of the literature on the epidemiological transition that was published since these seminal papers confirms the context-specific nature  of the epidemiological transition: while there is an overall all-cause mortality decline, the nature of cause-specific mortality declines differs across contexts. Increasing obesity rates in high-income countries are further confirming the epidemiological transition theory as the epidemic leads to an increase in NCDs. The picture is more nuanced in low- and middle-income countries, where there are signs of a protracted transition with the double burden of communicable and noncommunicable disease. A recent review of cause-specific mortality rates from 12 low- and middle-income countries in Asia and sub-Saharan Africa by Santosa and Byass (2016 ) shows that broadly, low- and middle-income countries are rapidly transitioning to lower total mortality and lower infectious disease mortality. A more macro-level analysis from the Global Burden of Disease data conducted by Murray and others (2015) finds that while there is a global trend towards decreasing mortality and increasing NCD prevalence, this global trend is being driven by country-specific effects as opposed to a broader transition; further, there are varying patterns within and between countries, which makes it difficult to have a single unified theory of epidemiological transition. A theory of epidemiological transition aimed at explaining not  just describing changes in population disease and mortality profiles would need to encompass the role in different morbid conditions of infectious diseases contracted over the life course. The concept of linear transition from infectious diseases to other conditions referred to as degenerative or non-communicable, was based on a false dichotomy as common microorganisms have now been confirmed as causal agents in several conditions recorded as the underlying cause of many deaths. A revised transition model might focus more on disease aetiology and the determinants of cause-specific mortality change, while encompassing the possibility that infectious causation may be established  for other morbid conditions through the vast amount of ongoing research into associations with infectious diseases.