Epidemic investigation lecture

1. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition (Digital).
EPIDEMIC
INVESTIGATION
Jeriel Reyes De Silos, MD, MPM-HSD
Department of Family and Community Medicine

2. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
Objectives:
At the end of the lecture, students should be able to:
Explain the different levels of disease;
Explain the three objectives of epidemic investigation;
Examine the different types of epidemic;
Given the initial information of a possible disease outbreak, describe
how to determine whether an epidemic exists;
Discuss the steps in investigating epidemics.

3. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
I. Levels of Disease

4. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
I. Level of Disease
Epidemic
Outbreak
Cluster
Pandemic
Endemic
Sporadic
Hyperendemic

5. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
I. Levels of Disease
Normal Range of Cases
I. Levels of Disease
Definition:
1. Epidemic:
• The occurrence in a community or
region of cases of illness, specific
health-related behavior or health-
related events clearly in excess of
normal expectation. (Last. A
Dictionary of Epidemiology, 4th ed.)
• “Unusually high disease occurrence in
an area.”
Date
Confirmed Cases

6. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
I. Levels of Disease
Normal Range of Cases
I. Levels of Disease
Definition:
2. Outbreak:
An epidemic limited to localized
increase in the incidence of a disease,
e.g., in a village, town, or closed
institution. (Last. A Dictionary of
Epidemiology, 4th ed.)
Date
Confirmed Cases

7. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
Cluster of dengue cases
I. Levels of Disease
I. Levels of Disease
Definition:
3. Cluster:
Aggregation of cases grouped in place
and time that are suspected to be
greater than the number expected.
(www.cdc.goc/epicasestudies.
Available: 2017)

8. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
I. Levels of Disease
I. Levels of Disease
Definition:
4. Pandemic:
An epidemic that has spread over
several countries or continents,
usually affecting a large number of
people. (www.cdc.goc/epicasestudies.
Available: 2017)

9. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
Normal Range of Cases
I. Levels of Disease
I. Levels of Disease
Definition:
5. Endemic:
• Constant presence of disease within a
geographical area (Last. A Dictionary of
Epidemiology, 4th ed.)
• Constant presence and/or usual
prevalence of a disease or infectious
agent in a population within a
geographic area
(www.cdc.goc/epicasestudies. Available:
2017)
• Examples: Malaria (Palawan),
Schistosomiasis (Samar-Leyte Islands)
Date
Confirmed Cases
Moving Average

10. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
I. Levels of Disease
I. Levels of Disease
Definition:
6. Sporadic:
• Intermittent presence of disease
within a geographical area. (Last. A
Dictionary of Epidemiology, 4th ed.)
• Disease that occurs infrequently and
irregularly.
• (www.cdc.goc/epicasestudies.
Available: 2017)
• Examples: Chikungunya, Alzheimer’s
disease
Date
Confirmed Cases
Sporadic cases

11. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
I. Levels of Disease
Normal Range of Cases
I. Levels of Disease
Definition:
7. Hyperendemic:
A disease that is constantly present at a
high incidence and/or prevalence and
affects most or all age groups equally.
(Last. A Dictionary of Epidemiology, 4th
ed.)
Hyperendemic – “constant”
Epidemic – “sudden”
Date
Confirmed Cases
Moving Average

12. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
II. Objectives of Epidemic
Investigation

13. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
II. Objectives of Epidemic Investigation
1. To determine the nature
of the disease and the
mode of its propagation;
2. To ensure that specific
control measures will be
applied;
3. To prevent further
transmission of the
disease- causing agents.

14. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern

15. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern
1. Common- source Epidemic
Intermittent
Continuous
Point-source
2. Propagated Epidemic
3. Mixed Epidemic

16. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern
1. Common-source epidemics:
Caused by exposure to a
COMMON NOXIOUS
INFLUENCE at the same time
Exposure period can be:
Brief (point-source)
Long (continuous)
Intermittent
Example: Food poisoning

17. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern
1. Common-source epidemics:
A. Point-source:
Happens when a group is exposed
to a disease source over a short
period.
Everyone who got the disease
does so within one incubation
period.
DISEASE
SOURCE
Close contact range

18. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern
1. Common-source epidemics:
A. Point-source:
Incubation period: The time
interval between the exposure of
a susceptible person to a
pathogen and the appearance of
signs and symptoms
Time (in DAYS)
Start of
infection
No infection
Incubation
Period
With Symptoms
Recovered
Hypothetical Infectious Disease

19. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern
1. Common-source epidemics:
A. Point-source:
Example: Leukemia after the
atomic bomb blast in Hiroshima,
Japan
Graph follows a “Log-Normal”
epidemic curve pattern (steep
upslope, gradual downslope)
Shape: PLATEAU
Source: Dr. Shinya Matsuura
Director, Research Institute for Radiation Biology and Medicine,
Hiroshima University

20. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
Date
Confirmed Cases
III. Types of Epidemic Pattern
1. Common-source epidemics:
B. Continuous common-source:
Cases do not all occur within a
single incubation period.
Epidemic curve downslope can
be:
Very sharp: if the common source is
removed or;
Gradual: if the epidemic is allowed
to exhaust itself.
Example: Cholera
Shape: PLATEAU
Common
source is
removed

21. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern
1. Common-source epidemics:
C. Intermittent common-source:
It has a pattern reflecting the
intermittent nature of the
exposure.
Epi curve shows an irregular
pattern of cases.
Date
Confirmed Cases

22. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern
2. Propagated epidemics:
Transmission of an infectious
agent from susceptible host to
another.
Direct person to person (ex: STIs)
Vehicle-borne (ex: Hepatitis C via
needles
Vector-borne (malaria)
Index case – first case in a
community
Cases occur over more than one
incubation period
Shape: Bell-shaped

23. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
III. Types of Epidemic Pattern
3. Mixed pattern:
Starts with a continuing
common- source exposure.
Followed by secondary
propagated exposure (person-
to-person).
Cases DO NOT develop within
one incubation period.
Shape - PLATEAU
Date
Confirmed Cases

24. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating
Epidemics

25. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures

26. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
QUESTION: What is the disease condition?
TOOL: CASE DEFINITION
• Standard set of criteria for deciding if a person is to
be classified as having the disease or not.
• Not the same as a CLINICAL DIAGNOSIS
ACTIVITIES:
• History, PE, Laboratory tests
• Compare with case definition
Step 1 is considered COMPLETED when the diagnosis has
been established (reported cases = TRUE cases).

27. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
QUESTION: Is there an epidemic?
TOOL: HISTOGRAM or LINE GRAPH
Used to show a TREND OVER TIME
• 𝑋-axis = time
• 𝑌-axis = number of cases
ACTIVITIES:
• Count the cases
• Compare with usual occurrence (baseline)
• Determine if current incidence is in excess of the
usual

28. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
Sources Of Data:
1. Morbidity reports
2. Mortality reports
3. Hospital discharge records
4. Interview of doctors/nurses
5. Electronic Health Records
Determine the usual frequency of disease
• In the same area
• Among a specified population
• At the same season of the year

29. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
Example:
• 200 Leptospirosis cases in Province X and Y
• Date observed: January, 2019
0
50
100
150
200
250
Jan-15 Jan-16 Jan-17 Jan-18 Jan-19
Dengue Cases in Province X and Y,
Jan 2015-2019
Province X Province Y
No Epidemic
Epidemic

30. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
Other explanations for a sudden increase in
the number of cases:
1. Changes in case definition
2. Improvements in diagnostics
3. Changes in the population size
4. Increased interest
5. Data error
6. Corrected data error
Step 2 is considered COMPLETED when it is
confirmed that the current incidence is
MARKEDLY IN EXCESS OF THE USUAL.

31. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
APPROACH: DESCRIPTIVE EPIDEMIOLOGY
• Trend over time
• The population affected by the disease
• Geographic extent of the epidemic

32. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO TIME:
QUESTIONS:
• What is the exact period of the epidemic?
• What is the type of epidemic?
• When was the probable period of exposure?
TOOL: EPIDEMIC TIME CURVE
• A histogram-type of graph
• 𝑋-axis = time of onset
• 𝑌-axis = number of symptomatic cases
Activities:
• Determine the type of epidemic curve
• Determine the probable period of exposure

33. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO TIME:
Activities:
1. Determine the type of epidemic curve.
• Explosive Curve

34. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO TIME:
Activities:
1. Determine the type of epidemic curve.
• Explosive Curve
• Bell- shaped Curve

35. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO TIME:
Activities:
1. Determine the type of epidemic curve.
• Explosive Curve
• Bell- shaped Curve
• Plateau

36. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO TIME:
Activities:
2. Determine the probable period of
exposure:
• METHOD 1: Mean incubation period
• METHOD 2: Minimum-maximum
incubation period

37. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO TIME:

38. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO TIME:

39. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PERSON:
QUESTIONS:
1. What are the age- and sex- specific attack
rates?
2. What age and sex groups are at the highest
and lowest risk of illness?
3. In what other ways do cases differ from the
non-cases?
TOOL: ATTACK RATE

40. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PERSON:
TOOL: ATTACK RATE
‫݇ܿܽݐݐܣ‬ ܴܽ‫݁ݐ‬ =
ܰ‫ݎܾ݁݉ݑ‬ ‫݂݋‬ ܰ݁‫ݓ‬ ‫ݏ݁ݏܽܥ‬
ܲ‫݊݋݅ݐ݈ܽݑ݌݋‬ ܽ‫ݐ‬ ܴ݅‫݇ݏ‬ ‫ݎ݋‬
‫݀݁ݏ݋݌ݔܧ‬ ‫݋ݐ‬ ܽ݊ ܱ‫݇ܽ݁ݎܾݐݑ‬
Activities:
1. Determine the age- and sex- specific attack rates
2. Determine specific attack rates for other
characteristics

41. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PERSON:

42. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PERSON:

43. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PERSON:
Activities:
1. Determine the age- and sex- specific attack
rates
2. Determine specific attack rates for other
characteristics
• Race
• Occupation
• Behavior
• Socio-economic status, etc.

44. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PLACE:
QUESTIONS:
1. What is the area- specific attack rate?
2. What is the most significant geographic
distribution of cases?
TOOLS:
1. Attack rate
2. Spot map

45. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PLACE:
ACTIVITIES:
1. Determine where cases live, work or may
have been exposed
2. Determine area- specific attack rates
3. Observe for CLUSTERING AND PATTERNS

46. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PLACE:

47. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CHARACTERIZE AS TO PLACE:
Step 3 is considered completed when
the characteristics as to person- place-
time provide a basis for the formulation
of hypothesis.

48. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
ANALYTIC EPIDEMIOLOGY:
1. Etiologic agent
2. Source of the agent
3. Period of exposure
4. Mode of transmission
5. Population at risk
TOOLS:
1. Analytic studies
2. Retrospective cohort
3. Case Control
ACTIVITY:
Compute for the measure of association

49. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures

50. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
Step 4 is considered
COMPLETED
when sources and mode of
transmission are identified.

51. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
EPIDEMIOLOGIC TRIAD
Agent
Host Environment

52. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
EPIDEMIOLOGIC TRIAD
Environment
Agent
Host

53. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
IV. Steps in Investigating Epidemics
1. Verify the diagnosis
2. Confirm the existence of
an epidemic
3. Characterize the
epidemic as to person-
place-time
4. Identify the etiologic
agent and mode of
transmission
5. Identify the population
at risk and institute
control measures
CONTROL MEASURES
1. Control the source of the pathogen (example:
improved sanitation)
2. Interrupt the transmission of the disease (example:
isolation/ quarantine, contact tracing)
3. Control or modify the host response to exposure
(example: vaccination)
Step 5 is considered COMPLETED when control measures
have been initiated.

54. Background: How some cities ‘flattened the curve’ during the 1918 flu pandemic. National Geographic September 2020 Edition
(Digital).
References:
• www.cdc.gov
• Last. A Dictionary of Epidemiology, 4th ed.
• National Geographic Magazine, September 2020 Issue.
• Katz, et al. Jekel’s Epidemiology, Biostatistics, Preventive Medicine and
Public Health, 5th ed. 2020
• Ben-Shlomo, et al. Epidemiology, Evidence-Based Medicine and Public
Health Lecture Notes, 6th ed. 2013
• Carnate, J.M. Epidemic Investigation Lecture. 2015