In demography and medical geography, epidemiological transition is a phase of development witnessed by a sudden and stark increase in population growth rates brought about by medical innovation in disease or sickness therapy and treatment, followed by a re-leveling of population growth from subsequent declines in fertility rates. "Epidemiological transition" accounts for the replacement of infectious diseases by chronic diseases over time due to expanded public health and sanitation. This theory was originally posited by Abdel Omran in 1971.
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:
- The Age of Pestilence and Famine: Where mortality is high and fluctuating, precluding sustained population growth, with low and variable life expectancy, vacillating between 20 and 40 years.
- The Age of Receding Pandemics: Where mortality progressively declines, with the rate of decline accelerating as epidemic peaks decrease in frequency. Average life expectancy increases steadily from about 30 to 50 years. Population growth is sustained and begins to be exponential.
- The Age of Degenerative and Man-Made Diseases: Mortality continues to decline and eventually approaches stability at a relatively low level.
The epidemiological transition occurs as a country undergoes the process of modernization from developing nation to developed nation status. The developments of modern healthcare, and medicine like antibiotics, drastically reduces infant mortality rates and extends 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.
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 third world 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 advancements in medicine and the devopment of a healthcare system. One treatment breakthrough of note was the discovery of penicillin in the mid 20th century which led to widespread and dramatic declines in death rates from previously serious diseases such as syphilis. Population growth rates surged in the 1950s, 1960s and 1970s, to 1.8% per year and higher, with the world gaining 2 billion people between 1950 and the 1980s alone.
Omran's third phase occurs when human birth rates drastically decline from highly positive replacement numbers to stable replacement rates. In several European nations replacement rates have even become negative. As this transition generally represents the net effect of individual choices on family size (and the ability to implement those choices), it is more complicated. Omran gives three possible factors tending to encourage reduced fertility rates:
- Biophysiologic factors, associated with reduced infant mortaliity and the expectation of longer life in parents,
- Socioeconomic factors, associated with childhood survival and the economic perceptions of large family size, and
- Psychologic or emotional factors, where society as a whole changes its rationale and opinion on family size and parental energies are redirected to qualitative aspects of child-raising.
This transition may also be associated with the sociological adaptations associated with demographic movements to urban areas, and a shift from agriculture and labor based production output to technological and service-sector-based economies.
Regardless, Chronic and degenerative diseases, and accidents and injuries, became more important causes of death. This shift in demographic and disease profiles is currently under way in most developing nations, however every country is unique in its transition speed based on a myriad of geographical and socio-political factors.
Many question whether or not epidemiological transition really took place during the twentieth century. The transition during this time describes the replacement of infectious diseases by chronic diseases. This replacement of diseases has been identified to be caused by multiple factors such as antibiotics and increased overall public sanitation. Even though these factors undeniably affected society in ways such as increased lifespan, many believe that the increase from infectious disease to chronic disease may be an illusion. It is debated that there was an actual increase in chronic diseases. Instead it is argued that due to new techniques of diagnosing and managing diseases that previously had been undiagnosed and untreated, it gave the appearance of an emergence of new chronic illnesses. Multiple factors made chronic diseases more visible to health care professionals such as increased use of hospitals as treatment centers and improved statistical evaluation. This led to the question, "Was an epidemiological transition really taking place in the twentieth century?"
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