1. Epidemiology- I
Mahmood ur Rahman

2. Definition of Epidemiology
Epidemiology is the study of the distribution and determinants of diseases or states in population groups and the application of this study to the control of health problems
(Last JM ed. Dictionary of Epidemiology, Oxford University Press, 1995)

3. 'Epidemiology is that field of medical science which is concerned with the relationship of various factors and conditions which determine the frequencies and distributions of an infectious process, a disease, or a physiologic state in a human community'
(Lilienfeld 1978)

4. Historical Perspective
Hippocrates - 5th century
Association between external environment and personal characteristics and health

5. “Whoever wishes to investigate medicine properly should proceed thus: in the first place consider the seasons of the year, and what effects each of them produces. Then the winds, the hot and the cold, especially such are as common to all countries, and then such as are peculiar to each locality. In the same manner, when one comes into a city to which he is a stranger, he should consider its situation, how it lies as to the winds and the rising of the sun; for it influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. One should consider most attentively the waters which the inhabitants use, whether they be marshy and soft, or hard and running form elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hallow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor.” (Hippocrates, “On airs, waters and places” Medical Classics 3:19, 1938).

6. Historical Perspective
John Graunt – 1662 (Hennekins and Buring 1987)
The Nature and Political Observations Made Upon the Bills of Mortality
Systematic statistical approach
Analyzed births and deaths in London
Excess of males born, higher mortality for males
Infant mortality is very high
Seasonal variation for mortality
Importance of routinely collected information for study of human illness
William Farr
Examined mortality and occupation and marital status
Identified important issues in epidemiological investigations
Use of comparison population, influence of multiple factors on disease

7. Historical Perspective
John Snow (1854) – Father of modern epidemiology
Established modern epidemiologic methods
Cholera epidemic in London
Plotted geographical location of all cases – deaths from cholera

8. From The Visual Display of Quantitative Data, Edward R. Tufte

9. John Snow (cont)
Went door to door, collecting information on daily habits
Suspected water supply as source of epidemic
Broad street pump closed, epidemic stopped
Mode of investigation – “shoe leather”
Practical application of epidemiology – use epidemiological investigation to impact a health problem

10. How the Epidemiologist Works
Studies origin and distribution of a health problem
Collection of data
Constructs a logical chain of inferences to explain the various factors in a society or segment of society that cause a health problem to exist
Likened to a detective investigating the scene of a crime looking for clues
Starts with examination of sick person(s)
Extends investigation to the setting where illness is occurring
Looks for common denominator that links all the affected so that the cause of the problem can be eliminated or controlled

11. Epidemiologic Analyses – Areas of Study
Causal agents related to disease:
Biological agents – bacteria, viruses, insects
Nutritional agents – diet (fats, carbohydrates, food nutrients)
Chemical agents – gases, toxic agents
Physical agents – climate, vegetation, chemical pollutants (air, water, food)
Social agents – occupation, stress, social class, lifestyle, location of residence

12. Epidemiologist studies:
Host characteristics:
Biological factors
Age, sex, degree of immunity, other physical attributes that promote resistance or susceptibility
Behavioral factors
Habits, culture, lifestyle
Social environment
Living conditions such as poverty, crowding
Norms, values and attitudes
Socially prescribed standards of living
Use of food and water, food handling practices
Household and personal hygiene

13. Eras of Epidemiology Sanitary era – early 19th century
Infectious disease era – between late 19th century and early 20th century
Chronic disease era – 2nd half of 20th century
Eco-epidemiology era – 21st century

14. Comparing the job of a clinician and the job of an epidemiologist
The clinician
Deals with patients
Takes a history
Conducts a physical
Makes a diagnosis
Proposes a treatment
Follows up the patient
The epidemiologist
Deals with populations
Frames the question
Investigates
Draws conclusions
Gives recommendations
Evaluates programmes

15. Person
Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004
Cases
Population
Attack rate per 1000
Age
0-4 1
1012
0.1
5-9 11
21802 2
10-14 37
74004 5
15-44
416
51358 81
45+ 73
56153 13
Sex
Male
341
102683
3.3
Female
197
101646
1.9

16. Uses of epidemiology Examine causation Study natural history
Description of the health status of population
Determine the relative importance of causes of illness, disability and death
Evaluation of interventions
Identify risk factors

17. 1. Examine causation Genetic factors Good health Ill health
Environmental factors
(Biological, chemical, physical, psychological factors)
Life style
related factors

18. To study historical rise and fall of disease in the population
The diseases wax and wane (tuberculosis ) new ones appear (encephalitis, asbestos poisoning, LSD psychosis) old ones are eradicated (smallpox)

19. Number of countries with smallpox, 1967-1978

20. A Pertussis by 5-Year Age Groups B
A Pertussis by 5-Year Age Groups B. Pertussis by <1, 4-Year, Then 5-Year Age Groups      

21. Lung Cancer Rates in the United States, 1930–1999

22. Infant Mortality Rates for 2002, by Race and Ethnicity of Mother

23. Place
Attack rate of acute hepatitis by zone of residence, Orissa, India, 2004
Attack rate
Underground water supply
Pump from river bed
/ 1000
/ 1000
/ 1000
20+ / 1000
Chipat river

24. Common Source (Vehicle), Single (Point) Exposure

25. Outbreak of Cholera, London, August-September , 1854

26. Common Source, Continued exposure :

27. Propogated source

28. Seasonal fluctuations :
Malaria and JE are commoner during immediate post monsoon season.
Asthma shows highest incidence during spring and autumn suggesting specific environmental factors in its causation

29. Secular trend

30. Searching for causes and risk factors
Theories in Primitive and Middle Ages
Supernatural Causes e.g. being possessed by evil spirits, wrath of gods, punishment for evil deeds during previous births “bad air”
Contagion Theory
William Farr -Miasma Theory
Germ Theory - Henle-Koch postulates
Epidemiological Wheel Theory
The Theory of “Necessary” and Sufficient” cause

31. Physical (injury, trauma)
Role of the host, the agent and the environment in the occurrence of disease
Biologic, Chemical,
Physical (injury, trauma)
Social Psychological
AGENT
VECTOR
Sanitation
Weather
Pollution
Socio-Cultural
Political
Genotype
Nutrition
Immunity
Behaviour
ENVIRONMENT
HOST

32. Causes of Tuberculosis

33. Syndrome identification
Epidemiologists are called “lumpers and splitters”
The differentiation of hepatitis A from hepatitis B and the distinction between several varieties of childhood leukemia.
“lumping” include the identification of many manifestations of tuberculosis.
Patterson-kelly syndrome of association between dysphagia and iron deficiency anaemia but when association was tested by epidemiological studies , it was not found.

34. Individual risk and chances: Life table-The ‘risks’ the average male in England

35. Measures of disease frequency
Prevalence
Number of cases of a disease in a defined population at specified point of time
Incidence
Number of new cases, episodes or events occurring over a defined period of time
Population at risk

36. Incidence rate I = Number of people who get the disease or condition
in a specified time
X Factor
I =
Total population at risk

37. Population at risk All women (age groups) Total population
0-25
years
25-69
years
25-69
years
Men
Women
70+
years
Eg. Population at risk in a study of carcinoma of cervix

38. Prevalence P = Number of people with the disease or condition
at a specified time
P =
X Factor
Total population at risk

39. How to remember this: think about a bathtub
Cure
incidence
prevalence
Death

40. Community diagnosis :
The definition of indicators is a pre-requisite Indicators - to estimate the burden of illness and the strategies for control.
The main health indicators are expressed in terms of crude age-adjusted or age-specific mortality rates (such as infant mortality rates, mortality for children under 5, or maternal mortality rates), disease-specific morbidity rates, and life expectancy at birth.

41. 3. Study natural history Death Sub-clinical disease Clinical disease
Good health
Recovery

42. Natural history of disease timeline

43. 4. Evaluation of interventions
Treatment, Medical care
Good Health
Ill Health
Health promotion
Preventive measures
Public health services

44. Evaluation
Evaluation is the process of determining, as systematically and objectively as possible, the relevance, effectiveness, efficiency, and impact of activities with respect to established goals.

45. Effectiveness refers to the ability of a program to produce the intended or expected results in the field; effectiveness differs from efficacy, which is the ability to produce results under ideal conditions.
Efficiency refers to the ability of the program to reproduce the intended results with a minimum expenditure of time and resources.

46. Descriptive Epidemiology
Collecting all data describing occurrence of disease
Person
Disease predominates in smokers
Disease predominates immuno-compromised individuals
Place
Disease is associated with an arid climate
Disease is associated with a tropical climate
Time
Disease associated with the date of company picnic
A particular year

47. Disease surveillance Frequency with which the disease occurs
Definition of the disease
Size of the population from which the cases develop
Completeness of the reporting of the cases.

48. Types of epidemiology Descriptive epidemiology
Distribution
− Frequency of health events
− By person, time and place
Descriptive epidemiology
Determinant
− Search for causes or risk factors
− Response to a study hypothesis
− Use various epidemiologic methods
Analytic epidemiology
Health, disease or injury
Disease-specific epidemiology
Application
Applied epidemiology

49. Epidemiologic triad
HOST
DISEASE
AGENT
ENVIRONMENT

50. Factors Associated with Increased Risk of Human Disease
HOST
(Intrinsic)
AGENTS
ENVIRONMENT (Extrinsic)
Age
Gender
Ethnicity
Religion
Customs
Occupation
Heredity
Marital status
Family background
Previous diseases
Biological (bacteria, etc.)
Chemical (poison, alcohol, smoke)
Physical (auto, radiation, fire)
Nutritional (lack, excess)
Temperature
Humidity
Altitude
Crowding
Housing
Neighborhood
Water
Milk
Food
Radiation
Air pollution
Noise

51. EPIDEMIOLOGY Measure of mortality and morbidity Rates Ratio Proportion
Scientific basis of public health
Measure of mortality and morbidity
Risk (Cumulative incidence) / Prevalence / Incidence
Rates
Ratio
Proportion

52. Ratio is one number divided by another number (numerator doesn’t have to be included in denominator—and vice versa)
Proportion is a ratio in which the numerator is included in the denominator
It has no dimension because the unit of the numerator cancels out the unit of the denominator
Rate is a ratio
The numerator is the number of events
The unit is event (i.e., number of cases)
The denominator is the sum of follow-up time contributed by the people at risk of the event
The unit is time or, more accurately, person-time to account for duration of time of follow-up