Epidemiology

Epidemiology is the study (scientific, systematic, data-driven) of the distribution (frequency, pattern) and determinants (causes, risk factors) of health-related states and events (not just diseases) in specified populations (patient is community, individuals viewed collectively), and the application of (since epidemiology is a discipline within public health) this study to the control of health problems.

Purposes of Epidemiology:

a)    Describing disease occurrence and distribution of disease in population groups and development over time (trend).

b)    Identifying cause and risk factors of disease (aetiology).

c)    Undertaking experiments to assess the effects of treatments or preventive efforts.

d)    Elucidating the natural history and prognosis of disease.

e)    Providing foundation for public policy.

Types of Epidemiology:

1)    Descriptive Epidemiology:

 Is the type of epidemiology which aims to study the distribution and determinants of diseases to describe variations in the frequency of diseases between population by geographical areas and over time (person, place, time).

Triad of descriptive epidemiology:

Time:

The occurrence of diseases changes over time, they can occur regularly or unpredictable. For diseases that occur seasonally, health officials can anticipate their occurrence and implement control and prevention measures, such as an influenza vaccination campaign. For diseases that occur sporadically, investigators can conduct studies to identify the causes and modes of spread, and then develop appropriately targeted actions to control or prevent further occurrence of the disease.

Displaying the patterns of disease occurrence by time is critical for monitoring disease occurrence in the community and for assessing whether the public health interventions made a difference.

Place:

Describing the occurrence of disease by place provides insight into the geographic extent of the problem and its geographic variation. Characterization by place refers not only to place of residence but to any geographic location relevant to disease occurrence. Such locations include place of diagnosis or report, birthplace, site of employment, school district, hospital unit, or recent travel destinations.

Person:

Because personal characteristics may affect illness, organization and analysis of data by “person” may use inherent characteristics of people (for example, age, sex, race), biologic characteristics (immune status), acquired characteristics (marital status), activities (occupation, leisure activities, use of medications/tobacco/drugs), or the conditions under which they live (socioeconomic status, access to medical care).

Age: A number of factors that also vary with age include: susceptibility, opportunity for exposure, latency or incubation period of the disease, and physiologic response.

Sex: Males have higher rates of illness and death than do females for many diseases.

Ethnic and racial groups: Sometimes epidemiologists are interested in analyzing person data by biologic, cultural or social groupings.

2)    Analytical Epidemiology:

Is the type of epidemiology where from observations of descriptive analysis, epidemiologists develop hypotheses about the causes of these patterns and about the factors that increase risk of disease.

The key feature of analytic epidemiology is a comparison group.

Analytic epidemiology is concerned with the search for causes and effects, or the why and the how. Epidemiologists use analytic epidemiology to quantify the association between exposures and outcomes and to test hypotheses about causal relationships.

A critical premise of epidemiology is that disease and other health events do not occur randomly in a population, and a number of models of disease causation have been proposed. Among the simplest of these is the epidemiologic triad.

The triad consists of an external agent, a susceptible host, and an environment that brings the host and agent together.

Agent: referred to an infectious microorganism or pathogen but Over time, the concept of agent has been broadened to include chemical and physical causes of disease or injury. These include chemical contaminants as well as physical forces.

Host: refers to the human who can get the disease. A variety of factors intrinsic to the host, sometimes called risk factors, can influence an individual’s exposure, susceptibility, or response to a causative agent.

Environment: refers to extrinsic factors that affect the agent and the opportunity for exposure. Environmental factors include physical factors such as geology and climate, biologic factors such as insects that transmit the agent, and socioeconomic factors such as crowding and sanitation.

Now, we will take about some concepts related to occurrence of diseases:

Sporadic refers to a disease that occurs infrequently and irregularly.

Endemic refers to the constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area.

Hyperendemic refers to persistent, high levels of disease occurrence.

Epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area.

Outbreak carries the same definition of epidemic, but is often used for a more limited geographic area.

Cluster refers to an aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known.

Pandemic refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people.

Comparison between Prevalence and Incidence:

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