the habitual presence of a disease within a given geographical area. The disease regularly exists in a 

geographic area

• levels of an endemic disease may vary over time but the disease remains present in a population

worldwide epidemic

• is an epidemic of infectious disease that has spread through human populations across a large region; for instance multiple continents, or even worldwide. A widespread endemic disease that is stable in terms of how many people are getting sick from it is not a pandemic.
• A disease or condition is not a pandemic merely because it is widespread or kills many people;it must also be infectious.

“The study of the distribution and determinants 

of health-related states or events in specified 

populations and the application of this study 

to control of health problems.”

• Determine the cause (etiology) of a disease
• Determine risk factors for the disease
• Determine the level of disease in the community
• Understand the natural history of the disease
• Develop and evaluate interventions and treatments
• Provide a basis for the development of public policy to promote good health and prevent disease

Non-Modifiable:

- Age

- Gender

- Race

Modifiable:

- Eating habits

- Exercise habits

- Obesity

Intervention with a healthy (well) person

 - Person does not yet have the disease in question

 - Want to prevent them from ever getting the 

disease

 - Immunizations against infectious diseases, 

- prevention of exposure to environmental risks

 Important goal but not always possible

Intervention in a diseased but asymptomatic 

person (preclinical disease)

 - Involves interventions such as screening 

programs for cancer

 - The goal is early detection and therefore a better 

potential outcome

 - Relies on the availability of a screening test

Reduce the impact of disease in someone who has 

already been diagnosed and is clinically ill

•  Involves treating an illness quickly and appropriately
•  Proper rehabilitation from the illness after treatment
•  Relies on available treatment

Prevention is applied to the population as a whole

•  Individual habits and risk factors are not considered
•  By necessity must be inexpensive and not too invasive

Examples: Smoke-free campus, newborn testing, mandatory immunizations

•  Targets a high risk group specifically
•  Less expensive but not as encompassing
•  May be more invasive or inconvenient

Ex. Early screening for colon cancer in people with a 

family history

Specific organisms cause specific diseases

•  Developed between ~1850 and 1900
•  Resulted from advances in science that allowed viruses and bacteria to be visualized, isolated, and used to cause disease in healthy animals

Miasmas – emanations from trash or decaying 

plant or animal matter that were believed to travel 

through the air and cause disease; early 1800’s (prior to Germ Theory)

• 
  Malaria is Italian for “bad air” and was thought to arise from swamps
• 
  Miasmas tied overcrowding to disease

Disease was caused by a chemical process of decay; early 1800’s

• Contact with rotting animal or vegetable matter transferred “zymes” that caused disease

• Involved the four principle elements: fire, water, air, and earth
•  In the body these elements were believed to be represented by four humoral fluids; Blood, phlegm, yellow bile, and black bile
•  Health was associated with a balance of humors
•  An imbalance of humors could be caused by travel, injury, diet, planetary alignment, etc.
•  Disease resulted from miasma/zymosis and humoral imbalance

Certain circumstances could make a disease 

contagious when at other times it was not

• “Seasoning” explained why people who were new to an area were more likely to become ill initially (until they became “seasoned”)

Contact that causes person to 

person spread of a disease

Ex. STDs, an ill person sneezing and then shaking hands with someone

Indirect – Disease is transmitted by a 

“vehicle”

Ex.

◦ Air - Tuberculosis, influenza (can also be fomite)

◦ Food or water - Salmonella (can also be direct or fomite), guinea worm

◦ Fomites (objects) - MRSA (can also be direct)

◦ Vector - West Nile Virus, Lyme Disease, Malaria

◦ The patient has signs and symptoms of 

disease

ex. Fever, pain, numbness, cough, sneezing, etc

◦ Signs and symptoms are not present

-  Can be preclinical, subclinical, persistent, or latent

◦ The patient has the disease but the signs and symptoms are not visible yet

◦ Preclinical disease will eventually progress to clinical disease

◦ Many cancerous tumors can exist for some time without causing signs and symptoms

◦ Left untreated, the tumor can grow/spread and cause signs and symptoms

◦ The patient has the disease but the signs and 

symptoms are not visible

◦ Subclinical disease will generally not eventually 

progress to clinical disease

◦ A virus or bacterium that remains in the 

system for a long period of time after initial infection

◦ May later cause disease, sometimes with 

different signs and symptoms from the initial disease

◦ Examples are Chicken pox and Shingles

The person is infected but the infectious 

agent does not multiply

◦ Often describes a food or waterborne outbreak 

◦ Many people are exposed to the same source of infection



Single or multiple exposures

 e.g., Food

 Periodic or continuous exposures

 e.g., water

you compare the risk of disease among groups who had different exposures

Equation:

Number of people at 

risk in whom a certain

illness develops               

Total number of people 

at risk

Number of people who ate a certain food and became ill

Total number of people who ate that food

Number Ill / Total Ate Food

Any person who becomes ill as a result of an 

outbreak and is included in a study is called a 

“case”

Primary - a person who gets the disease from 

the exposure

Secondary - A person who gets the disease from 

exposure to a primary case (can calculate a secondary attack rate )

Population growth

Urbanization

Poor disease transmission knowledge

Reservoirs

Travel

Nutrition 

Sanitation

Immunity

Vaccine-preventable

Preferably one dose for lifelong immunity

Infrequent virus mutations

No reservoir other than humans

Financial support and worldwide effort

Surveillance systems in place

(Small pox is the only successfully eradicated disease)

1. Prepare for field work 

2. Establish the existence of an outbreak 

3. Verify the diagnosis 

4. Define and identify cases 

5. Describe and orient the data in terms of time, 

place, and person 

6. Develop hypotheses 

7. Evaluate hypotheses 

8. Refine hypotheses and carry out additional studies 

9. Implement control and prevention measures 

10. Communicate findings

Disease occurrence

Measures:

- Incidence

- Prevalence

- Spot Maps

- Hospital Records

- Physician Records

- Self report (subjective)

- Health Insurance

- ICD 9 Codes

- DSM IV Codes

- ICF Codes

- 

Measurements of Disease Occurence

Rates - How fast disease is occurring in the population

Proportion - What part of the population is being affected

The number of new new cases that occur over a specific 

time period in a population at risk

# new cases in Gainesville in 2009    X 1,000

Population at risk in G’ville in 2009

◦ Measures new cases of disease

◦ Measures risk of developing disease

◦ Can be used for any group of people

◦ Generally multiplied by some arbitrary number 

(e.g., 100, 1000, 10,000)

◦ Denominator must include people at risk

Measures mortality for all causes (includes time)

total no. of death from all causes in 1 year      x 1000

No. of perople in the population at midyear

(If you put a restriction on a rate, it is called a specific rate)

ex. annual mortality rate from all causes for children younger than 10 y.o.

= no. of deaths from all causes in childs < 10y.o.x1000

   No. of children in populaiton younger than 10 y.o.

(percent)

no. of individuals dying during a

specific period of time after disease

onset or diagnosis                                  x 100

No. of individuals with the specified

disease

* Includes only people who are sick with the disease in the denominator (compare to mortality rate which includes all of the population at risk regardless of disease status)

(not a rate)

What proportion of all deaths caused by a condition?  Not risk

No. of deaths from X disease in the US in 2012   x 100

Total deaths in the US in 2012

Measures premature mortality

= predetermined age at death* - actual age at death

add all years up to ive a total YPLL for a condition

*in the US, this is 65 y.o.

Measures burden of disease and quantifies year of life lost to death and disease

DALY = 1 year of healthy life lost

                   Disease           No Disease

Positive          80(TP)               100 (FP)

Negative        20 (FP)               800 (TN)

Sensitivity = 80/100 = 80%      TP/TP+FP

Specificity = 800/900 = 89%     TN/TN+FP

True Positive (TP)

False Positive (FP)

True Negative (TN)

False Negative (FN)

(TP) = A person how HAS the disease and tests POSITIVE for the disease

(FP) = a person who does NOT have the disease but tests POSITIVE for the disease

(TN) = A person who does NOT have the disease and tests negative for the disease

(FN) = A person who DOES have the disease but tests NEGATIVE

Positive Predictive Value

Negative Predictive Value

How likely is it that they will test positive if they have the disease?

PPV - what proportion of patients who screen positive truly have the disease

NPV - what proporiton of patients who screen negative truly don't have the disease

                 Disease     No Disease

Pos             80                  100

Neg             20                  800

PPV = 80/100+80 = 80/180 = 44%

NPV = 800/20+800 = 800/820 = 98%

Characteristics of both prevalence of the disease in the population and the specificity of the screening test (if prevalence is low)