BASICS OF EPIDEMIOLOGY

1. Epidemiology

2. Patient Profile
• A 29 yr old previously health man referred to UCLA
with H/O Fever, Fatigue, LN enlargement and Wt loss of
almost 12.5 Kg in last 8 months.
• He had 39.50C temperature, appeared physically
wasted and had swollen LN
• Lab showed depressed Lymphocytes
•Pt suffered multiple simultaneous infections
•Candida albicans in upper GIT,
• Pneumocystis Carinii in lungs and
•Cytomegalovirus in urinary tract
• Not responding to antibiotics.
• No underlying cause of immuno-supression!!

3. • There were 3 other previously healthy young
males reporting within last 6 months with recent
H/O Wt. loss, fever, LN enlargement and all had
Candida albicans and Pneumocystis Carinii
infections
• What are the Similarities!!
• UCLA physicians notified public health officials &
prepared a Descriptive Report for Publication
• Between June and November 1981, - 76
instances of P. Carinii pneumonia were
identified in persons who did not have known
immunosuppression.

4. Personal Attributes
Age Early 30s
Sex Male
Prior health condition Good - health
Sexual preference Homosexual
Place of Occurrence Los Angeles
Time of Occurrence October 19, 1980 to June 19,
1981
Other groups who had higher occurrence of such disease
were Hemophiliacs' and Injecting drug users – Any relation?
Characteristics of 76 Sentinel Cases

5. Observational Data give Insight
Frequency of agents by age of children with pharyngitis
1.General Vision &
Wisdom of
Clinicians comes
from Population
Data.
2.Same hold for
Diagnosis and
Prognosis
3.RCTs - to Select
Appropriate
Therapy.

6. Ignáz Semmelweis and Childbed Fever

7. Maternal mortality due to childbed fever,
First and Second Clinics
Observation by Ignáz Philipp Semmelweis 1846

9. A painting by Gaston Melingue of Edward Jenner performing
the first vaccination in 1796.

10. AN EXPERIENCE

11. • John Snow (19th Cent.): Hypothesized that Cholera was
transmitted by contaminated water.
Water Supply No. of
Houses
Death from
Cholera
Death
/10,000
Houses*
Southwark & Vauxhall
Co.
Lambeth Co.
Other districts in
London
40,046
26,107
2,56,423
1,263
98
1,422
315
38
56
* Not a good Rate but good approximation
Remember that in Snow’s days “Vibrio cholerae” was unknown –
it is not always necessary to know every detail of Pathogenic
Mechanism to be able to Prevent a Disease.

12. An Epidemiologist…..
 Observe/ Imagine/ Doubt
 Investigate/ Analyze
Make Hypotheses
 Experiments
Tests Hypotheses

13. !
• How disease is distributed in
population
(What, Where, Who, When)
• What are the factors that determine
this distribution (Why, How).
• What can we do (What, How)

14. Epidemiology
Why does a disease develop in
some people and not in others?
The underling principle is that
disease is not randomly
distributed in population.
Certain characteristics
predispose or protect us against
a variety of different diseases.
15

15. Major Goal of epidemiology :
To intervene to Reduce Dis. Morbidity and Mortality
Objectives –
1. Identify the Cause of a disease and the Risk
Factor/s associated with it – Planning Prev./ Care
– Identify At Risk Sub-Groups (Screening Tests) to
identify Determinants of the Dis. and Transmission
Channels
2. Determine the Extent of disease found in the
community and Changing Patterns of Community
Health Problems) – Planning, Policy & Strategy
including Training

16. Changing Patterns of Community Health Problems

17. Health Policies and Strategies’ effect on Remaining yrs of Life

18. Objectives of Epidemiology
3. Study the Natural History and Prognosis
of Dis.– Planning Prev./ Care
4. Evaluate Preventive & Therapeutic
measures – Betterment of Care strategy/
system
5. Study Association of Environmental,
Genetic and Other Factors with human
health issues – Public Health Policy for
Prevention and Health Promotion 19

19. Epidemiologic Approach
Epidemiologic Reasoning - a Multistep
Process: (Comparable/ Quality Data)
First Step –
Determine whether an association exists
between exposure to a factor or a characteristic
of a person and the development of the disease
in question.
Second Step –
Find out Reasons for the Difference.
Find out if it is an Causal Association.

20. Relationship between rate of dental caries in children’s
permanent teeth and Fluoridation of water

21. Epidemiology often Begins with
Descriptive Data
VARIATIONS ARE INVESTIGATED
eg Caries Tooth
Inferences Made
! Certain level of Fluoride in water is Protective for Caries tooth
Ensure Comparable Data
Reasons for Variations
eg. Fluoride in Water

22. DMF indices after 10 years of fluoridation,

23. Effect of discontinuing fluoridation (after a referendum) in
Antigo, Wisconsin, November 1960. DMF

24. Epidemiology
World Health Assembly recognized the essential role of
epidemiology in “Global Strategy for HFA-2000”
Urged member states to use
-Epidemiological Data
-Concepts and
-Methods
For
Planning/ Preparation of health care activities
Monitoring/ Evaluation
And Updating their work

25. Definition:
“The study of distribution and
determinants of health related states or
events in specified populations
AND
Application of this study to control of
Health Problems.” 26
Epidemiology

26. 27
Health related State or events
Birth defects
Injuries
Reproductive health
Occupational Health
Environmental health
Epidemics / outbreaks
Endemic levels
Chronic diseases

27. 28
Distribution (Where, Who, When)
Time
Place
Person
Study – Disease or Health Event’s
Frequency
Rate
Pattern

28. 29
Determinants( What. Why & How)
Genetic predisposition
Life style & behavior
Environmental exposures
Demographic feature
Risk Factors – differentiate
Modifiable and Non-modifiable RFs

31. Some Basics

32. What is Disease?
• A condition where health is
impaired
• Departure from health
• A deviation in performance of
normal body functions
But this requires defining health
33

33. 34
How to define Health?
A state of Complete
Physical
Mental
Social Wellbeing;
Not merely the absence of disease or Infirmity
A positive phenomenon
A dynamic State

34. Human Disease
• Hosts must be susceptible
• Susceptibility determined by:
– Genetic background
– Nutrition
– Immunity (prior experience with natural infection
& immunization)
• Biological, physical and Chemical factors
may cause human disease
35

35. Probability of infection
D x Sc x T x V
Ip = ------------------------
Hd
Ip = probability of infection
D = dose
Sc = Susceptibility of host
T = time of contact
V = virulence
Hd = force of combined host
defenses or Immunity
36

36. AGENT
HOST ENVIRONMENT
EPIDEMILOGICAL TRIAD

37. • Interaction:
Agent
Host
Environment
• Chain of transmission
• Modes of spread
Agent Environment
Host
Why Do Diseases Occur ?

39. Clinical and Subclinical Disease
The “iceberg” concept of infectious diseases

40. Distribution of clinical severity for three
classes of infections

41. Nonclinical (Inapparent) Disease
Nonclinical disease may include the following:
1. Preclinical Disease- Disease that is not yet
clinically apparent but is destined to progress
to clinical disease.
2. Subclinical Disease- Disease that is not
clinically apparent and is not destined to
become clinically apparent. This type of
disease is often diagnosed by serologic
(antibody) response or culture of the organism

42. Nonclinical (Inapparent) Disease
3. Persistent (Chronic) Disease- A person fails to “shake
off” the infection, and it persists for years, at times for
life.
Post-polio syndrome in adult life. These have thus
become cases of clinical disease, albeit somewhat
different from the initial illness.
4. Latent Disease- An infection with no active
multiplication of the agent, as when viral nucleic acid is
incorporated into the nucleus of a cell as a provirus.
In contrast to persistent infection, only the genetic
message is present in the host, not the viable
organism.

43. Carrier Status
• An individual who harbors the organism but is
not infected as measured by serologic studies (no
evidence of an antibody response) or by evidence
of clinical illness.
• This person can still infect others, although the
infectivity is often lower than with other infections.
• May be of limited duration or may be chronic,
lasting for months or years.
– Typhoid Mary, who carried Salmonella typhi and died in
1938. Over a period of many years, she worked as a cook
in the New York City area, moving from household to
household under different names. She was considered to
have caused at least 10 typhoid fever outbreaks that
included 51 cases and 3 deaths.

44. Agent
Nature:
Biological
Physical
Chemical
Nutritional
Characteristics:
Infectivity
Pathogenicity
Virulence

45. Agent:
Infectivity :
Ability to invade and multiply
Pathogenecity:
Ability to produce clinically apparent disease
Virulence:
The degree of pathogenecity of an infectious agent,
indicated by case fatality rates and/or
its ability to invade and damage tissues of the host.

46. Reservoir of infection
• A case
– Clinical
– Sub-clinical
– Latent (host
does not
excrete agent)
• A carrier
– Types-
• Incubatory
• Convalescent
• Healthy
– Duration
• Temporary
• Chronic
– Portal of exit
• Urinary
• Intestinal
• Respiratory

47. Host Factors
• Physiological
• Age, Gender, Ethnicity
• Genetic predisposition
• Behavioral
• Food fads
• Life style
• Alcohol, Smoking, Exercise
• Hygiene-personal, sexual
• Education
• Occupation

48. Environment
Surrounding conditions/influences, (not part
of agent or host); could be-
Extrinsic
Intrinsic
Components:
Physical- geographical
Climatologic-Temperature,
Humidity Rainfall,
Biologic- Habitat, Life forms,
Vectors, Pests
Economic
Social
Political

49. Risk Factors
• Unidentified agent, exposure to which
has a significant bearing on
development of disease;
• or a determinant modification of which
can reduce possibility of disease
development.
• They are present either in individual,
family, community or environment; and
could be identified even before the
predicted outcome.

50. Risk factors could be-
1. Causative
2. Contributory
3. Predictive,
OR
1. Modifiable
2. Immutable/ un-modifiable

51. Study of Natural History of Disease
Answers
How to control Diseases?
What can prevent a Disease?
What is the prognosis?
Risk factors?
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52. • Positive Health
• Good Health
• Freedom from Sickness
• Unrecognized Sickness
• Mild Sickness
• Severe Sickness
• Death
53
SpectrumofHealth
Health is a process of continuous change

53. 54
Diagnosis
Therapeutic
Interventions
Risk modification
Health
promotion
Vaccines
Screening
Recovery
with or
without
DisabilityHealth Disease Death
Natural History of Disease AND what can I do ?
Rehabilitation
Subclinical Phase
Clinical Phase
Biological onset

54. Latent
Subclinical
undiagnosed
Ice Berg Phenomenon

55. 56
Early
Pathogenesis-
Tissue changes
Tissue
changes
Pre-
Pathogenesis-
A, H, E
interaction
Pathogenesis
Levels of
Prevention
Primary Secondary
Tertiary
Health.
Promotion
Specific
Protection
Susceptibility
Pre-
symptoma
tic stage Clinical disease
Early
diagnosis
Treatment
Disability Limitation/
Rehabilitation
Death
Recovery
Disability
Chronic state
Convalescence
Preven-
tion.
Modes
Immunity &
Resistance

56. Stages of Disease
• Clinical disease –
Signs and Symptoms Present
• Non clinical (in-apparent) disease:
Preclinical disease
Sub clinical disease
57

57. Stages of Disease
–Persistent (chronic) disease –
persistent infection, sometimes is
permanent
–Latent disease – genetic message is
present, not the viable organism
58

58. Carrier:
An individual (host-human or animal) who
harbors a microorganism (agent) without
evidence of disease and, in some cases,
without evidence of host immune response; &
serves as potential source of transmission.
 May be Healthy carrier it can be for short
duration or May be chronic (e.g.Typhoid Mary)
 An incubatory carrier, or
 A convalescent carrier.

59. Reservoir:
Any person, animal, arthropod, plant, soil
or substance or combination of these;
in which an infectious agent normally
lives and multiplies,
on which it depends primarily for survival,
And
where it reproduces itself in such manner
that it can be transmitted to a susceptible
host.

60. Infectious Agent:
An organism that is capable of
producing infection or infectious
disease
Virus,
Bacteria,
Rickettsia,
Fungus,
Protozoa or
Helminthes

61. Communicable disease:
An illness due to a specific infectious agent
or its toxic products which arise through
transmission of that agent or its product
from an infected person, animal or
inanimate reservoir to a susceptible host
either directly or indirectly.
Infectious disease:
A clinically manifest disease of man or
animal resulting from an infection

62. • Primary case is the first case that has occurred
• Secondary case is a person/s who acquire disease
from primary case
• Index case is the first case brought to the notice of
Health system
• Serial Interval The gap in time between onset of
Primary case and Secondary case (s); useful in
situations where incubation period is not known
• Attack Rate No. of at risk people developing disease
Total No. of at risk people
• Secondary Attack Rate is Attack rate in susceptible
people who have been exposed to primary case

63. Extent of Disease
• Endemic: habitual presence of a disease within a
given geographic area
• Epidemic: occurrence in a community or region of
a group of illnesses of similar nature, clearly in
excess of normal expectation, and derived from a
common or from a propagated source
• Pandemic: A global epidemic
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