Chapter 1

The History and Scope of Epidemiology

Learning Objectives

• Define the term epidemiology• Define the components of epidemiology (determinants,

distribution, morbidity, and mortality)• Name and describe characteristics of the epidemiologic

approach• Discuss the importance of Hippocrates’ hypothesis and

how it differed from the common beliefs of the time• Discuss Graunt’s contributions to biostatistics and how

they affected modern epidemiology• Explain what is meant by the term natural experiments,

and give at least one example

Epidemiology Defined

• Epidemiology derives from "epidemic," a term which provides an immediate clue to its subject matter. Epidemiology originates from the Greek words, epi (upon) + demos (people) + logy (study of).

Definition of Epidemiology

• Epidemiology is concerned with the distribution and determinants of health and diseases, morbidity, injuries, disability, and mortality in populations.

• Epidemiologic studies are applied to the control of health problems in populations.

Key Aspects of This Definition

• Determinants• Distribution• Population• Health phenomena • Morbidity and mortality

Determinants

• Factors or events that are capable of bringing about a change in health.

Examples of Determinants

– Biologic agents--bacteria– Chemical agents--carcinogens – Less specific factors--stress, drinking,

sedentary lifestyle, or high-fat diet

The Search for Determinants

• Anthrax outbreak• Outbreak of Fear--Ebola virus in Kikwit,

Zaire• Fear on Seventh Ave.--Legionnaires’

disease in New York City• Red Spots on Airline Flight Attendants--

dye from life vests• Bioterrorism-Associated Anthrax Cases

Distribution

• Frequency of disease occurrence may vary from one population group to another.

Disease Distribution Examples

• Hypertension more common among young black men than among young white men.

• Coronary heart disease occurrence differs between Hispanics and non-Hispanics.

Population

• Epidemiology examines disease occurrence among population groups, not individuals.

• Epidemiology is often referred to as population medicine.

• The epidemiologic description indicates variation by age groups, time, geographic location, and other variables.

Health Phenomena

• Epidemiology investigates many different kinds of health outcomes:– Infectious diseases– Chronic diseases – Disability, injury, limitation of activity – Mortality – Active life expectancy– Mental illness, suicide, drug addiction

Morbidity and Mortality

• Morbidity--designates illness.• Mortality--refers to deaths that occur in a

population or other group. • Note that most measures of morbidity and

mortality are defined for specific types of morbidity or causes of death.

Aims and Levels

• To describe the health status of populations.

• To explain the etiology of disease.• To predict the occurrence of disease.• To control the occurrence of disease.

Foundations of Epidemiology

• Interdisciplinary• Methods and procedures—quantification• Use of special vocabulary• Epidemic frequency of disease

Epidemiology Is Interdisciplinary

• Epidemiology is an interdisciplinary field that draws from biostatistics and the social and behavioral sciences, as well as from the medically related fields of toxicology, pathology, virology, genetics, microbiology, and clinical medicine.

Quantification

• Quantification is a central activity of epidemiology.

• Epidemiologic measures often require counting the number of cases of disease.

• Disease distributions are examined according to demographic variables such as age, sex, and race.

Epidemic

• “The occurrence in a community or region of cases of an illness (or an outbreak) clearly in excess of expectancy…”

• Relative to usual frequency of the disease.

Infectious Disease Epidemics

• A single case of a long absent communicable disease.

• First invasion of a communicable disease.

• Two cases of such a disease associated in time and place are sufficient evidence of transmission to be considered an epidemic.

Concept of Epidemic and Non-Infectious Diseases

• Some examples that use the concept of an epidemic are:– Love Canal – Brown lung disease– Asbestosis among shipyard workers– Diseases associated with lifestyle

Pandemic

• “ . . . an epidemic on a worldwide scale; during a pandemic, large numbers of persons may be affected and a disease may cross international borders.” An example is a flu pandemic.

Ascertainment of Epidemics

• Surveillance• Epidemic threshold

Surveillance

• The systematic collection of data pertaining to the occurrence of specific diseases.

• Analysis and interpretation of these data.• Dissemination of disease-related

information.

Epidemic Threshold

• The minimum number of cases (or deaths) that would support the conclusion than an epidemic was underway.

Historical Antecedents

• Environment and disease• The Black Death• Use of mortality counts• Smallpox vaccination• Use of natural experiments• Identification of specific agents of disease• The 1918 influenza pandemic

The Environment

• Hippocrates wrote On Airs, Waters, and Places in 400 BC.

• He suggested that disease might be associated with the physical environment.

• This represented a movement away from supernatural explanations of disease causation.

Use of Natural Experiments

• John Snow was an English physician and anesthesiologist.

• He investigated a cholera outbreak that occurred during the mid-19th century in Broad Street, Golden Square, London.

Snow’s Contributions

• Linked the cholera epidemic to contaminated water supplies.

• Used a spot map of cases and tabulation of fatal attacks and deaths.

Snow’s Natural Experiment

• Two different water companies supplied water from the Thames River to houses in the same area.

• The Lambeth Company moved its source of water to a less polluted portion of the river.

• Snow noted that during the next cholera outbreak those served by the Lambeth Company had fewer cases of cholera.

Natural Experiment

• Definition: The epidemiologist does not manipulate a risk factor but rather observes the changes in an outcome as the result of a naturally occurring situation.

Contemporary Natural Experiments

• Currently, natural experiments may be the result of legislation, policy changes or environmental interventions.

Examples of Contemporary Natural Experiments

• Seat Belt Law--Did seat belt use reduce fatalities from motor vehicle accidents?

• Tobacco Tax--Did the increase in cigarette price decrease the sale of cigarettes?

• Helmet Law--Did requiring the use of helmets by motorcyclists reduce the number of head injuries sustained?

Recent Applications of Epidemiology

• Framingham Heart Study (since 1948) investigates coronary heart disease risk factors.

• Smoking and lung cancer; e.g., Doll and Peto’s study of British doctors’ smoking.

• AIDS, chemical spills, breast cancer screening, secondhand smoke.

Additional Applications of Epidemiology

• Infectious diseases– Avian influenza

• Environmental health• Chronic diseases• Lifestyle and health promotion• Psychiatric and social epidemiology• Molecular and genetic epidemiology