Key Concepts
The occurrence of cases of disease in excess of what is usually expected for a given period of time. An epidemic is a breakout of disease affecting unusually large numbers of people within a community or population. When an epidemic spreads throughout manifold countries or continents, it is termed a pandemic (see illustration). In general, epidemics involve outbreaks of acute infectious disease, such as measles, polio, or streptococcal sore throat. In addition, other types of health-related events, such as homicide, drownings, and opioid abuse, have been considered to occur as "epidemics." See also: Disease; Infectious disease
Confusion sometimes arises as a result of overlap between the terms epidemic, outbreak, and cluster. Although they are closely related, epidemic may be used to suggest problems that are geographically widespread, whereas outbreak and cluster are reserved for problems that involve smaller numbers of people or are more sharply defined in terms of the area of occurrence. For example, an epidemic of influenza could involve an entire state or region, whereas an outbreak of gastroenteritis might be restricted to a nursing home, school, or day-care center. The term cluster may be used to refer to noncommunicable disease states.
In contrast to epidemics, endemic problems are distinguished by their consistently high levels over a long period of time. As mentioned previously, a pandemic is closely related to an epidemic, but it is a problem that has spread over a considerably larger geographic area; influenza pandemics are often global.
Origins of epidemics
Disease and epidemics occur as a result of the interaction of three factors: agent, host, and environment. Agents cause the disease, hosts are susceptible to it, and environmental conditions permit host exposure to the agent. Understanding of the interaction between agent, host, and environment is crucial for the selection of the best approach to prevent or control the continuing spread of an epidemic.
The importance of the agent-host-environment relationship is exemplified by the problem of epidemic influenza, which can result in high attack rates, excessive illness levels, and substantial mortality among certain population groups. The agent for epidemic influenza is the highly infectious influenza A or B virus. The host is someone who has never before been infected by the specific strain of influenza virus and lacks protective antibodies to it. If the environment is a confined one, such as a nursing home, the efficiency of virus transmission from a source, most likely another person with a case of influenza, to the susceptible host is heightened. See also: Antibody; Influenza; Virus
For infectious diseases, epidemics can occur when large numbers of susceptible persons are exposed to infectious agents in settings or under circumstances that permit the spread of the agent. Spread of an infectious disease depends primarily on the chain of transmission of an agent: a source of the agent, a route of exit from the host, a suitable mode of transmission between the susceptible host and the source, and a route of entry into another susceptible host. Modes of spread may involve direct physical contact (for example, touching or sexual intercourse) between the infected host and the new host, or airborne spread (for example, via coughing or sneezing). Indirect transmission takes place through contaminated water, food, or intravenous fluids; inanimate objects, such as bedding, clothes, or surgical instruments; or a biological vector, such as a mosquito or flea.
Responses to epidemics
Detection of epidemics is dependent on public health surveillance systems that monitor the incidence of different types of diseases. Surveillance for infectious disease epidemics or outbreaks relies heavily on reports of disease cases by physicians, hospitals, day-care centers, and other sources for health information. Regional and local public health agencies bear primary responsibility for investigating disease outbreaks, implementing control measures to halt their spread, and conducting long-term follow-up monitoring to detect recurrences. See also: Public health
When potential outbreaks are detected, a public health agency may initiate an epidemic investigation. The investigation is a systematic procedure that combines aspects from the disciplines of epidemiology, clinical medicine, laboratory science, and communications. The steps involved in an epidemic investigation help to determine who became ill, where the individuals were affected, and when they were affected. By examining the person, place, and time features, the investigator may be able to explain why the epidemic occurred and to identify appropriate control measures. However, because of the urgency associated with many epidemics, public health officials may need to take steps to control the problem while the investigation continues. See also: Epidemiology; Medicine
The primary purpose of an epidemic investigation is identification of the source or mode of disease transmission in order to recommend control measures. However, investigations of epidemics are conducted for other reasons. They include evaluation of intervention effectiveness, epidemiologic research in situ, and on-site training of public health workers and others engaged in epidemic control. In addition, such studies help fulfill the requirements of specific disease-control programs and provide more-immediate information in response to political or public concerns.
