1. Epidemiology

3. Epidemiology Comes from Greek words epi, meaning “on or upon”
demos,meaning “people”
logos, meaning “the study of”
Study of distribution and determinants of health-related conditions or events in populations

4. History
Hippocrates (circa 400 B.C.) attempted to explain disease occurrence from a rational instead of a supernatural viewpoint
John Graunt, a London haberdasher, published his landmark analysis of mortality data in 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.”
Discipline did not flourish until the end of the Second World War

6. Population or community health assessment
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?

7. The Epidemiologic Approach
Case Definition – a set of standard criteria for deciding whether a person has a particular disease or other health-related condition.

8. 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
Health departments convert the case counts into rates, which relate the number of cases to the size of the population where they occurred

9. Types of Epidemiology

10. 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
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

11. Person
Inherent characteristics of people
Age
Race
Sex

12. Acquired Characteristics
Immunity
Marital status

13. Activities Occupation Leisure activities
Use of medications/tobacco/drugs

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

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

16. Experimental
Determine the exposure status for each individual (clinical trial) or community (community trial)
Follow the individuals or communities to detect the effects of the exposure

17. Observational
Observe the exposure and outcome status of each study participant
Cohort study - categorize subjects on the basis of their exposure and then observe them to see if they develop the health conditions being studied
Case-control study - enroll a group of people with disease (“cases”) and a group without disease (“controls”) and compare their patterns of previous exposures

18. Causation

19. Causes
Cause 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

20. Epidemiologic Triangle
Traditional model of infectious disease causation
Agent
Host
Environment

21. Agent Agent Host Environment
Infectious microorganism - must be present for disease to occur
Virus
Bacterium
Parasite
other microbe
Agent
Host
Environment

22. Host Factors Agent Host Environment
Intrinsic factors that influence an individual’s exposure, susceptibility, or response to a causative agent.
Age
Race
Sex
socioeconomic status
behaviors
Agent
Host
Environment

23. Environmental Factors
Agent
Host
Environment
Extrinsic factors which affect the agent and the opportunity for exposure
physical factors
biologic factors
socioeconomic factors

24. Epidemiology and Disease

25. Chain of Infection
Reservoir
Mode of
Transmission
Susceptible
Host

26. Chain of Infection
Reservoir of an agent is the habitat in which an infectious agent normally lives, grows, and multiplies
Portal of exit is the path by which an agent leaves the source host

27. 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 (vector) or inanimate (vehicle) intermediaries
Airborne
Vehicleborne
Vectorborne
Mechanical
Biologic

28. Chain of Infection
Portal of entry – means by which an agent enters a susceptible host
Host – individual infected with the agent

29. Epidemic Disease Occurrence
Level of Disease - amount of a particular disease that is usually present in a community
Sporatic
Endemic
Epidemic
Pandemic
Increasing amount of disease

30. Epidemic Disease Occurrence
Sporadic - irregular pattern of occurrence, with occasional cases occurring at irregular intervals Endemic - persistent level of occurrence with a low to moderate disease level Epidemic or Outbreak- occurrence of a disease within an area is clearly in excess of the expected level for a given time period Pandemic - an epidemic spreads over several countries or continents, affecting a large number of people