Epidemiology is the study of the distribution and determinants of health conditions in populations. It has its roots in ancient Greece but developed as a scientific discipline in the mid-1800s due to systematic data collection of mortality statistics. Epidemiology is used for population health assessment and investigating disease causation through descriptive and analytical approaches. Key concepts include agents, hosts, environments, and the chain of infection in disease transmission.
2. E p id e m io lo g y
Comes from Greek words
e p i , meaning “on or upon”
d e m o s ,meaning “people”
l o g o s , meaning “the study of”
Study of distribution and determinants of
health-related conditions or events in
populations
3. History
Hippocrates (circa 400 B.C.) attempted to explain
disease occurrence from a rational instead of a
supernatural viewpoint
4. John Graunt, a London haberdasher, published his
landmark analysis of mortality data in 1662. He
was the first to quantify patterns of birth, death,
and disease occurrence, noting male-female
disparities, high infant mortality, and seasonal
variations
5. Mid 1800’s
William Farr began to systematically collect and
analyze Britain’s mortality statistics.
John Snow, an anesthesiologist, conducted a
series of investigations in London that later
earned him the title “the father of epidemiology.”
6. discipline did not flourish until the end of the
Second World War
8. P o p u la t io n o r
c o m m u n it y h e a lt h
a s s e s s me nt
What are the actual and potential health problems
in the community?
Where are they?
Who is at risk?
Which problems are declining over time?
Which ones are increasing or have the potential to
increase?
How do these patterns relate to the level and
distribution of services available?
9. T h e E p id e m io lo g ic
Approa c h
C a s e D e f i n i t i o n – a set of standard
criteria for deciding whether a person has a
particular disease or other health-related
condition.
10. Statistics and Reports
When physicians diagnose a case of a reportable
disease they send a report of the case to their local
health department.
Time - when the case occurred
place - where the patient lived
person - the age, race, and sex of the patient
11. Health departments convert the case counts into
r a t e s , which relate the number of cases to the
size of the population where they occurred
13. Descriptive Epidemiology
organize and summarize data according to time,
place, and person
Time – disease rates change over time – the
seasonal increase in influenza with the onset of
cold weather
time data is usually shown on a graph
14. Place - describe a health event by place to gain
insight into the geographical extent of the
problem.
Residence
Birthplace
place of employment
school district
hospital unit, etc
15. Person
inherent characteristics of people
Age
Race
Sex
20. E x p e r im e n t a l
determine the exposure status for each individual
(clinical trial) or community (community trial);
we then follow the individuals or communities to
detect the effects of the exposure
21. O b s e r v a t io n a l
observe the exposure and outcome status of each
study participant
c o h o r t s t u d y - categorize subjects on the
basis of their exposure and then observe them to see if
they develop the health conditions being studied
c a s e -c o n t r o l study - enroll a group of people
with disease (“cases”) and a group without disease
(“controls”) and compare their patterns of previous
exposures
23. c a u s e of disease is a factor (characteristic,
behavior, event, etc.) that influences the
occurrence of disease
An increase in the factors leads to an increase in
disease.
Reduction of the factors leads to a reduction in disease
24. E p id e m io lo g ic
t r ia n g le
traditional model of infectious disease causation.
Agent
Host Environment
25. Ag e nt
Infectious microorganism - must be present for
disease to occur
Virus
Bacterium Agent
Parasite
other microbe
Host Environment
26. Host factors
Intrinsic factors that influence an individual’s
exposure, susceptibility, or response to a causative
agent.
Age Agent
Race
Sex
socioeconomic status
behaviors
Host Environment
27. Environmental factors
Extrinsic factors which affect the agent and the
opportunity for exposure
physical factors
biologic factors Agent
socioeconomic factors
Host Environment
29. Chain of Infection
Mode of
Transmission
Susceptible
Reservoir Host
Mode of
Transmission
30. Chain of Infection
r e s e r v o i r of an agent is the habitat in
which an infectious agent normally lives, grows,
and multiplies.
P o r t a l o f e x i t is the path by which an
agent leaves the source host
31. Chain of Infection
Modes of transmission
Direct – immediate transfer of the agent from a reservoir to a
susceptible host by direct contact or droplet spread
Direct contact
Droplet spread
Indirect – an agent is carried from a reservoir to a susceptible
host by suspended air particles or by animate (v e c t o r ) or
inanimate (v e h i c l e ) intermediaries
Airborne
Vehicleborne
Vectorborne
Mechanical
Biologic
32. Chain of Infection
Portal of entry – means by which an agent
enters a susceptible host
Host – individual infected with the agent
33. Epidemic D i s e a s e
O c c urre nc e
Level of Disease - amount of a particular
disease that is usually present in a community
Increasing amount of disease
Pandemic
Epidemic
Endemic
Sporatic
34. Sporadic - irregular pattern of occurrence, with
occasional cases occurring at irregular intervals
35. Endemic - persistent level of occurrence with a
low to moderate disease level
36. Epidemic or Outbreak- occurrence of a disease
within an area is clearly in excess of the expected
level for a given time period
37. Pandemic - an epidemic spreads over several
countries or continents, affecting a large number
of people