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.

1. Epidemiology

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

7. Uses

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

12. Types of Epidemiology

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

16. Acquired Characteristics
 Immunity
 Marital status

17. Activities
 Occupation
 Leisure activities
 Use of medications/tobacco/drugs

18. Conditions under which people live
 Socioeconomic status
 Access to medical care

19. Analytic Epidemiology
 Used to search for causes and effects, or the why
and the how.

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

22. Causation

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

28. Epidemiology and Disease

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