Epidemics and Plagues

Epidemiology is the study of the distribution of diseases in populations and of factors influencing the occurrence of disease. Epidemiologists study both epidemic (excess) and endemic (always present) diseases. The basis of epidemiological science rests on the premise that disease does not occur by chance but rather that a range of environmental [Page 409]and personal characteristics that vary within populations influence disease occurrence. Epidemiological studies, which include those seeking to explain the transmission of communicable diseases by determining commonalities among those who become ill, are an important basis for public health interventions and policies. In recent decades, epidemiologic studies have focused on a fuller understanding of factors influencing chronic conditions (e.g., diabetes, heart disease, and cancer) that are prevalent in developed countries. Studies of these factors have been successful in identifying personal characteristics and environmental exposures that increase the risk of disease, thereby contributing to preventative measures and public policy.

Epidemics

An epidemic may be defined as the temporary increase of cases of disease in a geographically defined area in excess of what is usually expected on the basis of recent experience. The general public associates epidemics with infectious diseases (also called “contagious” or “communicable” diseases) that are caused by pathogens (diseasecausing organisms), which include bacteria, viruses, fungi, protozoa, parasites, and prions. The term epidemic derives from the Greek terms epi (upon) and demos (people). Because an epidemic depends on the “usually expected” number of cases of a disease within a population, even one case of a very rare or unexpected disease (e.g., smallpox) may represent an epidemic whereas many cases of a disease (e.g., malaria) would be classified as endemic rather than epidemic.

Other terms often associated with epidemic include outbreak, cluster, and pandemic. A cluster is a grouping (or aggregation) of cases of a disease or other health-related events or conditions that are related by time and place. Outbreak is a term used to describe a large number of cases of disease in a short period of time among a localized group of people. Clusters of sufficient size and importance may be considered outbreaks. A pandemic is an epidemic that is widespread and affects a whole region, a continent, or the world (e.g., AIDS). Other terms often found in epidemiological reports are prevalence, incidence, and rate. Prevalence is the number of diseased individuals (cases) at any one time (point prevalence) or over a given period (period prevalence). Incidence refers to the number of new cases of a disease, injury, or other condition that occur within a defined population over a given period of time. Rate may be thought of as the number of cases in a fixed number of people, for example, cases per 100,000.

Epidemics are caused by a common source of infection, such as food or water. Common-source epidemics can cause illness in large numbers of persons. Examples of common-source epidemics include botulism (from soil-contaminated food), cholera (from fecal-contaminated food and water), typhoid fever (from fecal-contaminated food and water), and brucellosis (from milk or meat of animals infected with Brucella spp.). Host-to-host infections, on the other hand, are transmitted from one infected individual to another through direct contact or through a carrier. Examples of host-tohost epidemics include tuberculosis (from inhalation of airborne particles), measles (from human sources), and hantavirus pulmonary syndrome (from inhaled fecal material of carrier rodents). Host-to-host epidemics tend to grow and to abate more slowly than common-source epidemics, because the latter type of epidemic generally stops once the source of infection has been identified and removed.

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