Epidemiology Variables: Time, Place, Person

Study Notes

Descriptive epidemiology involves characterizing disease occurrence by time, place, and person factors to identify associations and hypotheses about disease sources and routes of transmission
This step is crucial in outbreak investigations to develop control measures and prevent further cases

Epidemic curves are used to characterize disease occurrence over a short period of time for a specific population event
Epidemic curves are histogram graphs that display the number of cases over time
The shape of the epidemic curve can indicate the type of outbreak, such as a point source outbreak or a propagated outbreak
Point source outbreaks have a sharp rise and decline, while propagated outbreaks have a more gradual increase and decrease in cases
The incubation period of a disease can be determined from the epidemic curve, and knowing the incubation period can help identify the time of exposure

Seasonal cycles are repeating patterns of disease occurrence over a specific period of time
Seasonal cycles can be caused by vectors, diet, temperature, and other environmental factors
Secular trends are long-term patterns of disease occurrence over time
Secular trends can be caused by changes in disease reporting, case definition, or diagnostic techniques
Examples of seasonal cycles include foodborne diseases, leptospirosis, and norovirus
Examples of secular trends include changes in cancer rates over time

Characterizing disease occurrence by place involves identifying spatial patterns of disease distribution
Place can be defined as country, state, county, city, neighborhood, household, or other geographic unit
Spatial distribution can be influenced by population density, age distribution, breed or race distribution, exposure to occupations, and vectors
John Snow's study on cholera in London is a famous example of using spatial distribution to identify the source of a disease outbreak

Characterizing disease occurrence by person involves identifying demographic patterns of disease distribution
Person factors include age, sex, occupation, and behavior
Person factors can influence disease occurrence and transmission
Examples of person factors include the distribution of disease among different age groups or occupations### Disease Spatial Distribution
The difference in disease spatial distribution is related to soil type, level of moisture, and other environmental factors.

Person factors can include: age, sex, race, ethnic group, marital status, occupation, economic group, religion, habits, personal habits, travel history, disease symptoms, immunization status, and previous diseases.
These factors can provide important clues as to why particular individuals are affected and others are not, and how transmission is occurring.

Characterizing disease by epidemiology variables (time, place, and person) can yield important information about disease causation and suggest ways to control the disease.
This is true at the national, international, and local levels (e.g., specific food production facility, restaurant).

Brucellosis cases tend to be male and of middle age.
The age and gender distribution of brucellosis cases provided clues as to how individuals were coming to be infected (e.g., occupational exposure to infected animals).

Disease occurrence usually involves an interaction of the host, agent, and environmental factors.
Good descriptive epidemiology of time, place, and person factors is essential to understand how a disease can be transmitted and spread in a particular facility.