Audio Lecture   Module 1   Take the Quiz

Icon key

$ Read  

O Consider 

8 Link 

@ Do   

Introduction to Epidemiology

Instructions: Read the recommended text materials. Review the notes in this module.  Consider the questions or issues posed within the module.  Link to the supplemental sites for further information.  Take the quiz for a self-assessment. Do the discussion and study questions.


Define epidemiology.

Understand, recognize, and explain the scientific principles behind population-based observational studies.

Demonstrate familiarity with causation models.  

$ Recommended Reading:  Gordis, L. (2000). Epidemiology. W.B. Saunders, Philadelphia. Chapters 1 & 2.

 Key terms:

Epidemiology defined Scientific Premise of Epidemiology Epidemiologic Approach Causation Models of Disease Chain of Infection
Reservoir Vehicle (Transmission) Host Epidemiological Triad Agent
Environment Wheel of Causation Necessary Sufficient Web of Causation



Introduction to Epidemiology

Epidemiology defined: "the study of the distribution and determinates of health related states in specified populations and the application of this study to control health problems."

Predetermined or chance occurrence? – Determinism verses Stochastic models.

Underlying premise of epidemiology: "disease, illness, and ill health are not randomly distributed in a population."

 The complexity of nature precludes the knowing of all relevant factors except in the most simple of systems; therefore we must use probability models based on the best current available information. This means that absolute prediction is not possible, but we may know the likelihood of an event happening.  

Specific objectives of epidemiology:

Other uses of epidemiology:

Epidemiology and Clinical Practice: Diagnosis, Therapy, and Prognosis are all based on the use of epidemiological data about the disease and the individual's risk factors.

Epidemiological Approach: A search for associations between risk factors and disease; then to derive appropriate inference about causal relationships.

Historical Examples:

  (Return to top)

Causation Models (Not in readings)

 Chain of Infection.


Consider tuberculosis applied to the chain of infection.  The reservoir is an infected individual who is shedding the bacterium in his sputum or from expelled fluids.  Transmission is by cough or other contact with expelled fluids.  Host is a susceptible individual.  

 Epidemiological Triad (Triangle).


For TB:  In this model the agent is the TB bacterium; host factors include non-immune, weakened resistance, poor nutrition; and environmental factors include crowded conditions,  poor ventilation, and bad sanitation.

 Web of Causation: "Web" of interconnected factors which lead to disease.


TB in this model:  The factors of exposure to the agent, poor nutrition, crowding, poverty, low immunity, and concurrent disease may interact directly or indirectly to cause disease.

Wheels of Causation.


TB again:  One sufficient wheel might include: contact with a carrier, low immunity, and crowding; another wheel might include contact with a carrier, having AIDS, poor hygiene.  Both wheels are sufficient to produce disease, but factors differ.  But for TB, both wheels must have the TB organism, is a necessary factor for TB to occur.

Additional terms to understand:

Who? Where? When?

Person, place, and time factors are in fact the determinants we wish to understand.  Are they associated with an outcome?  Then is there evidence of a causal association?  If the evidence supports a causal association, then a factor is said to be a risk factor for that outcome.   From our evidence, we can often quantify the increased risk and determine (at least from a population standpoint) the risk reduction that can be achieved if that risk factor is removed.  

We will begin quantifying risk in our next module. 

Epi Supercourse-Intro to Epi: part 1     Epi Supercourse-Intro to Epi: part 2

More on "What is Epidemiology"

(Return to top)

Quiz:  This short quiz tests your general understanding of the material from this module.  The quiz is not graded, it is here for your self-assessment.  It is only a sample of representative content, so do not use the quiz as a lesson study guide.  Without referring to the notes above, you should miss none of these questions with superior understanding of the material.  If you miss over 2 questions on the first try, review the assignment.    

1.  A disease model that works best with infection diseases is  ______.

    a. web of causation    b. wheels of causation    c. epidemiologic triangle    

d. Koch's posulates    e. chain of infection   

2.  A disease model which discriminates  between 'necessary' and 'sufficient' factors is  ______.

    a. web of causation    b. wheels of causation    c. epidemiologic triangle    

d. Koch's posulates    e. chain of infection  

3.  An example of secondary prevention is  _______.

 a. disinfection   b. vaccination    c. disease screening     d. personal hygiene    e. environmental  sanitation 

4.   Epidemiology  uses  _________ in assigning associations and risk.

 a. germ theory   b. toxological data    c. number theory     d. determinism    e. probability theory 

5.    ________ data, including comparison groups or controls, is used in epidemiology to study associations and  disease determinants.

 a. Population    b. Ecological    c. Individual    d. Group    e. Environmental 

Assignment #1: 

  1. Define Epidemiology. (Review?)

  2. What is the underlying premise of Epdemiology? (Review?)

  3. What are the two principle elements of the epidemiologic approach? (Review?)

  4. Use the disease AIDS and explain it with each of the four causation models. (Review?)


(Return to top)