In demography and medical geograph

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INTRODUCTION

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."[1] 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.

Pestilence and Famine: Mortality is high and fluctuating, precluding sustained population growth, with low and variable life expectancy vacillating between 20 and 40 years. It is characterized by an increase in infectious diseases, malnutrition and famine, common during the Neolithic age. Before the first transition, the hominid ancestors were hunter-gatherers and foragers, a lifestyle partly enabled by a small and dispersed population. However, unreliable and seasonal food sources put communities at risk for periods of malnutrition by Santosa and Byass (2016

The Age of Receding Pandemics: 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. Mortality is increasingly related to degenerative diseases, cardiovascular disease (CVD), cancer, violence, accidents, and substance abuse, some of these due primarily to human behavior patterns. The average life expectancy at birth rises gradually until it exceeds 50 years. It is during this stage that fertility becomes the crucial factor in population growth by Samuel H,(1993).

In 1998 Barrett et al.[7] 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 Age of Declining CVD Mortality, Aging and Emerging Diseases: Technological advances in medicine stabilize mortality and the birth rate levels off. Emerging diseases become increasingly lethal due to antibiotic resistance, new pathogens like Ebola or Zika, and mutations that allow old pathogens to overcome human immunity by  Porta, (2014).

The Age of Aspired Quality of Life with Persistent Inequalities: The birth rate declines as lifespan is extended, leading to an age-balanced population. Socioeconomic, ethnic, and gender inequalities continue to manifest differences in mortality and fertility.

The theory of epidemiological transition uses patterns of health and disease as well as their forms of demographic, economical and sociological determinants and outcomes.

The epidemiological transition theory can be traced into three stages namely,

The age of pestilence and fimine.

The high and fluctuating precluding sustained population growth with low and variable life expectancy faciliting between 20 and 40 years. It is characterized by an increase in infectious, disease, malnutrition and famine, common during the Neolithic age.

Before the first transition, the hominid ancestors were Hunter-gatheres and foragers, a lifestyle partly enabled by a small and dispersed population.

The age of receding pandemics:

mortality progressively declines with the rate of decline accelerating as epidemic peaks decrease in frequency. Increases steadily from about 30 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.mortality is increasingly related to degenerative diseases, cardiovascular disease cancer, violence, accident and substance abuse. Some of those due primary to behavior patterns. The average life expectancy at birth rises gradually untill it exceeds so years. It is during this stage that fertility becomes the crucial factors in population growth by McGarvey, (2004).

There are several factors involved in the epidemiological transition. The most important are considered bellow;

       Demographic changes; Demographic changes are a composite of changes in both mortality and fertility. As populations become healthier, a reduction in mortality, particularly of infants and children, usually occurs, followed later by a fall in fertility rates. Therefore, more people will survive to adulthood and will have the disease patterns of adults, with non communicable diseases at the top of list. They will also be exposed to diseases that more frequently affect elderly people, such as cancer and cardiovascular diseases. Thus, even with constant age-specific incidence rates of noncommunicable diseases, the absolute number of cases and deaths from these diseases increases substantially with the above-mentioned demographic change.

     Biological factors; It is well known that microorganisms constantly undergo changes that enable them to cope with an increasingly hostile environment. In fact, the development of mechanisms that permit survival of the most adaptable microorganisms is more rapid than the development of defense mechanisms that allow their hosts to combat microbial invaders. This adaptive process involves finding and exploiting weaknesses in the defense of the host and can happen by means of several mechanisms: alteration in antigenic identity, emergence of drug-resistant strains and dual infection by Wahdan, (1996).

      Environmental factors; There is conclusive evidence that certain changes in the patterns of diseases, in particular a decrease in the occurrence of communicable diseases such as cholera, are the result of the development of environmental sanitation, particularly a clean water supply, sanitary disposal of waste and proper housing. On the other hand, environmental factors may increase the incidence of infectious diseases if they offer opportunities for transmission of etiological agents from the reservoirs of infection to susceptible hosts, for example by promoting breeding of vectors of diseases or because of overcrowding.

        Social, cultural and behavioral factors; Social, cultural and behavioral factors are closely related and interlinked. The shift from an agricultural to an industrial society and its accompanying process of modernization produce changes that affect people's health Modernization has adversely affected close community ties, which used to provide opportunities to share sorrow and happiness and to alleviate stress. An evident example of maladjustment due to urbanization is the traumatic encounter of rural youth with urban values. This frequently has a serious impact on mental health.Another example of the change in community relations is the care of the elderly. In rural communities where extended family and tribal life used to predominate, care of the elderly was essentially a family responsibility.

Conclusions

Although mortality differentials persist between socio-economic groups within high- and low-income countries, human beings in general have benefited from improvements in child survival and life expectancy. Public health and preventive measures aimed at protecting people from severe infectious diseases are likely to have contributed most to the decline in mortality, and the ongoing global epidemiological transition is fundamentally linked with efforts to control infectious diseases. Apart from setbacks in materially poor or remote populations affected by severe infectious diseases, and in countries undergoing rapid political, economic and social change, human intervention has resulted in a transition characterised by lower child mortality, an upward shift in age at death, increased life expectancy, a higher proportion of older people in the population and a reduction in the proportion of deaths for which an infectious disease is recorded as the underlying cause.

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