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Introduction to Epidemiology

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nick DrGS
country India
languages English, Hindi, Nepali


Experience: 3 years

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Epidemiology – The basic sciences of Public Health

Epidemiology is the study of the distribution of health outcomes or disease within populations.


It is also the factors that determine spread of health outcomes and diseases.  These factors are known as risk factors.


Epidemiologists focus their attention on the population level rather than on the individual level.


Early Epidemiology focus was on Infectious diseases till the 20th century. By 1950’s focus was shifted to conditions associated with infectious diseases. By 1960’s epidemiology started focusing on distribution of diseases. By 1988, epidemiology focused on control of health problems and diseases.


It is the most fundamental basic science of public health.


Epidemiological Transition - This describes change in pattern of populations in various age distributions, mortality, fertility, life expectancy and causes of death.


The human populations has gone through 4 major disease transitions:


  1. Emergence of Infectious diseases and diseases related to nutritional deficiencies. This is linked to beginning of agricultural and food productions 10,000 years ago. – This lead to incidence of zoonotic diseases, nutrient deficiencies, and increases contact with diseases vector during agricultural activities.
  2. Second transition was a time when human immune system and disease causing organisms both evolved resulting in a change from major epidemic of the disease to endemic of the disease. People developed physical and genetic changes to minimize the effects of diseases.
  3. The third transition occurred when disease pattern changed from infections to chronic and degenerative diseases in developed parts of the world due to improvement in nutrition, public health and clinical medicine. Cardiovascular and cancer started to occur more frequently during this transition. These are often associated with longer lifespan and sedentary lifestyle.
  4. The fourth transition started at the end of the 20th century when both new diseases and re-emergence of infectious diseases occurred and also due to rapid spread of disease due to globalization.  


Pioneers of Epidemiology

Hippocrates was a physician who wrote a book “On air, water and spaces” in which he defined epidemiology. He talked about environmental and behavioral issues associated with disease. He focused on rational explanations for disease.  He also defined Malarial parasite periodicity and malarial paroxysm. He also promoted swap drainage. Disease affects populations.


Girolamo Fracastoro – He explained the several ways in which transmission of several diseases can occur which can be by direct contact, air and infected clothing. He said that rapid multiplying seeds caused diseases.


Captain John Graunt - He was the first demographer. Demography is the statistical study of human populations. He presented the populations and mortality statistics at the time of the plaque is 1636. He did an analysis of the vital statistics of people living in London and published, “ Natural and political observations made upon the bills of mortality” in 1662.


James Lind – A Scottish physician, he was interested in the cause and treatment of Scurvy seen in the sailors of 16th and 17th century. He theorized that citrus fruits could cure scurvy. He took a leading step in the modern day clinical trails.


Percivall Pott – He was a British physician who showed that an environmental carcinogen could cause cancer. He noticed scrotal sores in the chimney cleaners, which has soot. His work was the beginning of non- infectious disease epidemiology.


William Farr – Considered one of the founders of modern epidemiology. He used statistical data to test social hypothesis. He also classified the causes of death n a broader aspect that it associated with health. He also showed a relationship between population density and mortality rates. He also mapped deaths, monitored outbreaks and developed categorization system for the causes of deaths.


Sir Edwin Chadwick – He studied sanitations issue in the United Kingdom. He emphasized that it is important to improve public health and social reform to improve health. He proposed fresh clean water, water closets i.e. flushing toilets in every home and proper sewage carrying system. He suggested watertight pipes for drinking water to avoid contamination with sewage water. He published his ideas in “the Sanitary Condition of the Laboring Population (1842).


John Snow – He is the father of modern epidemiology. He conducted the first outbreak investigation in London in 1954. He mapped cases of cholera depending upon where people worked and lived. He is the first person who counted and mapped cases geographically of a disease. His showed that contaminated water was the source of cholera. His work is considered as the beginning of epidemiology, which is the study of health outcomes in population.


Examples of Research areas using epidemiology


Infectious Disease Epidemiology – It is an epidemiological study of infectious and communicable diseases.

Chronic Disease Epidemiology – This field came in due to development of antibiotics and vaccines, which could take care of Infectious diseases. This change started in 1940’s and 1950’s


Maternal and Child health epidemiology focuses on development of health of the women, children and families.


Injury and Accidents Branch started developing in the late 1960’s.


Environmental epidemiology focuses on environmental risk factors, which affect public health. It includes chemical and physical agents, microbiological pathogens, social conditions and climate change.


Nutritional epidemiologists examine the association between nutrition and health outcomes. The food allergies rose by 18% from 1197 to 2007, by CDC, US. 3 million kids under the age of 18 had some kind of food or digestive allergies in the past 12 months.


Health policies can be made from the knowledge gained from epidemiological studies and their health outcomes. These policies should be evidence based. Health policies could be medical research policy, pharmaceutical policy, vaccine policy, tobacco control policy and breast-feeding promotion policy.  They also do research on substance abuse, psychiatric disorders, and communicable diseases such as TB, HIV and other sexually transmitted diseases.



Epidemiology is the study of the distribution and determinants of disease or health outcomes in specified populations, and the application of this study to the control the health problems.


Public Health is a multi-disciplinary filed whose goals is to promote the health of the population through organized community efforts.

Public health has the following features:

  1. Protects the populations against threat to health,
  2. Empowers people to lead healthy lives,
  3. Improves the quality of health services and
  4. Prepares leaders to advance health.


Epidemiology plays a role in public health in two aspects:

  1. Prevention - Epidemiology is used to develop, target, and evaluate prevention strategies. There are 3 levels of Public Health Prevention:

(a)  Primary prevention – This is before the person gets the health outcomes  (or disease) and tried to prevent the health outcome from occurring. It may reduce the prevalence, risk and rate of a health outcome. E.g. Vaccination.

(b)  Secondary Prevention occurs after the disease has occurred but before the person notices symptoms. It aims to find and treat the disease early, which cures the disease. E.g. checking for suspicious skin growths to rule out skin cancer.

(c)  Tertiary prevention is for people who already show symptoms of the disease. Its aim is to prevent the damage from the disease, slow down disease progression, prevent complications, provide better care and help people recover. E.g. provide dietary advice to help manage Diabetes.

  1. Interventions – They occur at two levels:

(a)  Individual – It focuses on changing individual risk factors and behaviors. E.g. educating individuals on the benefits of quitting smoking.

(b)  Structural – Changing the physical, social and/or economic factors in the environment to improve public health. E.g. providing clean drinking water, improving road safety standards, changes in car designs or change in urban design to promote more physical activity. A structural prevention to prevent smoking might be to increase the taxation of tobacco products to make them affordable. They depend upon an assumption that public health problems have some social causation.


Public Health is:

  • Preventing illness and disability and prolonging life
  • Promoting physical and mental health and efficiency
  • Community efforts for sanitation of the environment.
  • Prevention of diseases and injuries
  • Education of the individual.
  • Organization of services for the diagnosis and treatment of disease
  • Promoting a standard of living adequate for the maintenance of health.


Person, Place, Time and population perspective

 Descriptive Epidemiology refers to the characteristics of persons such as age, sex, race and socio-economic status, characteristics of place such as city, state, province, pin- code, and characteristics of time such as long term, short term, weekly or daily.

Descriptive Epidemiology also deals with the frequency and distribution of the diseases or risk factors in the population. It can be used for hypothesis generation but cannot be used to test the hypothesis. It can also be used to evaluate trends and disease to determine whether a health status is improving or getting worse, to determine if new diseases are occurring, and providing evaluation of public health problems for effectiveness. It can be used to identify problems to be studies by analytical epidemiology.


Analytical epidemiology aims to study and research the risk factors and preventive factors for diseases. It finds the underlying cause for the pattern of a disease. Use of a comparison group is a key feature of the analytical epidemiology. It searches for the sources of the causes and effects and the why and how of the causes of health outcomes.


Biomedical Perspective – The biomedical perspective focuses on the agents that cause the disease, their mechanism of action, and individual risk factors for all the health outcomes. It does not include social and environmental factors.


Population perspective – It focuses on social, psychological and environmental factors associated with health outcomes. We look two types of population perspective:

(a)  Proximal (downstream) – These are micro level factors which include physiological and biological factors such as genetic makeup, age, ethnicity, gender and immune status in relation with the disease. These are used to develop target interventions.

(b)  Distal (upstream) – These are macro level factors such as global forces and government policies. These help identify the factors at the source and hence they don’t have to be tackled downstream. These factors such as community design, education, employment, living and working conditions and poverty and major determinants of health outcomes.


Underlying causes of health outcomes in populations:

-       Environmental

-       Economic

-       Social

-       Cultural


In terms of population perspective, we have to talk about Web of Causation that underlying various factors contributing to a particular disease. Factors include family support, social network, global climate, labor conditions, tobacco productions and advertising. It also deals with social disparity related to socio-economic status.


How do health problems develop?

They can develop in many different ways.

1. Infections (Virus/bacteria)

2. Exposure to toxic chemicals

3. Genetic makeup may lead to improper gene sequence.

4. Behaviors/Lifestyles

5. Accidents like head trauma

6. Social factors (poverty/education)


E.g. Lung Cancer: Biomedical mechanism –


  1. Carcinogens in Tobacco Smoke
  2. Addictive properties of nicotine make it difficult to quit smoking
  3. Social and cultural factors