Epidemiology—Introduction

| May 22, 2015

The study of epidemics is epidemiology. Its primary focus is on the distribution and causes of disease in populations. Epidemiology involves developing and testing ways to prevent and control disease by studying its origin, spread, and vulnerabilities.

As a discipline, epidemiologic research addresses a variety of health-related questions of societal importance. Epidemiologic research methods are used by clinical investigators and scientists who conduct observational and experimental research on the prevention and treatment of disease.

The Cholera epidemic, a case from the 19th century, was enabled by the global movement of people. Having appeared in India in 1817, it spread throughout Asia and the Middle East within a decade. It was reported in Moscow in 1830 and then spread to Warsaw, Hamburg, Berlin, and London in 1831 (Snow, 1855, 2002). When it crossed the Atlantic to reach North America, Cholera gained the notoriety of the first truly global disease.

The modern day world is dominated by free trade and rapid transportation. An unprecedented rate of global interchange of food, consumer products, and organisms—including humans—is occurring. The threat of pandemics in the 21st century has heightened the importance of epidemiology at national and international levels.

Although diseases such as Influenza A (H1N1), Severe Acute Respiratory Syndrome (SARS), Acquired Immunodeficiency Syndrome (AIDS), West Nile Virus, Salmonella, are commonly recognized as epidemics, as they cause large scale disruption of health in populations. The field of epidemiology also addresses epidemics of obesity (Ogden et al., 2007), diabetes (Zimmet, 2001), mental health (Insel & Fenton, 2005), and any other disease that may cause large scale disruption of health in populations.

In general, there are ten stages to an outbreak investigation:

  1. Investigation preparation
  2. Outbreak confirmation
  3. Case definition
  4. Case identification
  5. Descriptive epidemiology
  6. Hypothesis generation
  7. Hypothesis evaluation
  8. Environmental studies
  9. Control measures
  10. Information dissemination

Investigation preparation requires a health crisis manager to identify a team of professionals who will lead the outbreak investigation, review the scientific literature, and notify local, state, and national organizations of the potential outbreak.

Outbreak confirmation requires actual laboratory confirmation of the disease, which may involve the collection of blood, urine, and stool samples from ill people and performing bacteriologic, virologic, or parasitic testing of those samples.

Case definition is the process by which we establish a set of standard criteria to determine who is and is not infected with respect to a specific outbreak; that is, a protocol is developed to determine case patients.

Case identification requires the health crisis manager and team of professionals to conduct a systematic and organized search for case patients based on the standard criteria developed in the case definition phase. Here, it is vital that the team identifies both case patients (individuals already infected), as well as individuals who may have been exposed.

Descriptive epidemiology requires the team to organize the data—who was infected, when, where, etc.—identify frequencies, and perform basic descriptive statistics that may present patterns.

Hypothesis generation requires the team to generate hypotheses as to why the outbreak occurred and compare the situation with previous outbreaks or develop methods to collect additional data.

Hypothesis evaluation requires more in depth statistical tests, including an official epidemiologic study (cohort, case control, etc.) and comparing potential risk factors among those infected (cases) and those not infected (controls).

Additional studies may call for environmental diagnoses requiring collection and analysis of food, water, and other environmental samples. Once all data has been collected and examined, the team will work to implement control and prevention measures by coordinating with community partner organizations.

Finally, the health crisis management team will communicate their findings via an outbreak investigation report that is tailored for the scientific community as well as lay persons. The primary goal of the information dissemination stage is to educate the community about the outbreak and instill the knowledge needed to prevent further outbreaks.

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