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Epidemic

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Epidemic Definition

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To understand what is an epidemic disease, let us go through the epidemic definition first. It is a disease that temporarily becomes highly prevalent over a specific geographical area. An epidemic occurs when a susceptible host and an agent are present in adequate numbers and the agent can be transmitted from a source to a susceptible host effectively. An epidemic disease may result from the following conditions:

  • It results from a recent increase in virulence or amount of the transmitting agent.

  • There is a recent introduction of an agent in an area where it was not previously found.

  • The agent or disease has an enhanced mode of transportation which results in more susceptible people getting exposed to the disease.

  • There is a change in the susceptibility of the host response to the agent.

  • There are factors that increase host exposure which might involve the introduction of the agent through new portals of entry.

We all know Covid-19 is a pandemic but have you ever thought about what is meant by pandemic? Or, what is an epidemic disease definition and how is it different from a pandemic disease? 


The amount of diseases that are present in a community is referred to as the endemic or baseline level of the disease. This is the observed level and not the desired level (which could in fact be zero). The disease might continue to occur at a particular endemic level without any intervention, provided it does not threaten to deplete the population that is susceptible to the disease. Therefore the baseline level of a disease is also its expected level. 


Though there are some diseases that are very rare and their occurrence (even a single case) deems an epidemiological investigation (for example, polio, rabies, plaque, etc.), there are other diseases that occur normally and warrant investigations only if there is any deviation from the norm. In this regard, there are a few terms used to depict the frequency or spread of diseases.

  • Sporadic - If a disease occurs irregularly and not frequently then its occurrence is called sporadic.

  • Endemic - The usual or constant presence of an infectious agent or disease in a specific geographical area.

  • Hyperendemic - This refers to a high level and persistent disease occurrence.

  • Epidemic - A sudden increase in the number of cases of a disease that is above what is expected normally in a given area.

  • Outbreak - This is similar to an epidemic but used for a smaller geographical area.

  • Pandemic - This is an epidemic that spreads over many different countries or continents, and affects a very large population.

Here, we will look into epidemic meaning, what is meant by pandemic, and the difference between epidemic and pandemic. We will also look into some of the world epidemics and pandemics.

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Epidemic Classification 

At the end of an epidemic, the affected population has a very small population of the susceptible host so that a reintroduction of the agent will not cause another epidemic. The reason being the host population is not conducive for the parasite population to reproduce itself as the host population has gained herd immunity and is not prone to the epidemic disease. An epidemic can be classified based on how it spreads among a population.

  • Common-source 

When the infectious agent or toxin from the same source affects a group of people then that is a common-source epidemic. It is further classified into 3 categories:

  • Point-source Outbreak - When the group of people is exposed for a relatively short period and all exposed people fall ill within the same incubation period then it is a point-source outbreak. Few examples of the point-source epidemic are leukemia cases in Hiroshima after the atomic bomb blast, hepatitis A, an epidemic in a restaurant in Pennsylvania where all those who ate green onion had a point source exposure.

  • Continuous - When the range of incubation period and range of exposures tend to flatten in a common-source endemic it is referred to as a continuous outbreak.

  • Intermittent - An intermittent common source reflects the intermittent nature of the exposure.

  • Propagated 

This outbreak results from direct person-to-person contact, i.e., it is transmitted from one person to another through contact for example syphilis. Transmission could also be vehicle-borne(for example HIV being transmitted through shared needles) or vector-borne (for example yellow fever being transmitted through mosquitoes). In a propagated outbreak, cases occur across incubation periods and usually wane after a few generations. 

  • Mixed 

A few epidemics resemble both common-source as well as propagated epidemics and are called mixed epidemics. It is not uncommon to have a pattern of a common-source outbreak which is followed by a secondary person-to-person transmission of disease. For example, shigellosis affected 3000 women who were attending a national music festival (common-source), and many of them developed symptoms after they returned home. In the ensuing weeks, several state health departments reported generations of Shigella cases which were transmitted person-to-person by those who attended the function.

  • Other 

There are a few epidemics that do not fall under the common-source or the propagated category. There could be an outbreak of vector-borne or zoonotic disease as a result of sufficient prevalence of infection in host species, presence of sufficient vectors, and ample human-vector interaction. An example is the epidemic of Lyme disease in the late 1980s in the northeastern part of the US (spread by deer tics from deers to humans).


Difference Between Epidemic and Pandemic

Epidemics defines a problem that has gone out of control and is defined as an outbreak of a disease over a large geographic area affecting a high proportion of the population. The WHO pandemic definition is “an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people”. 


An epidemic event is defined by an actively spreading disease while a pandemic describes geographical spread affecting the whole country or the world. The image below sums up the major difference between an epidemic and a pandemic.

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FAQ (Frequently Asked Questions)

Q1. What Does the Word “Epizootic” Mean?

Ans: Epizootic comes from two Greek words epi (meaning upon) and zoon (meaning animal) and it is a disease event seen in the animal population. Epizootic is analogous to epidemics in humans. Epizootics could be restricted to a specific area (called an outbreak), or widespread (called a panzootic). An example of epizootic is RVF (Rift valley fever) which primarily affects livestock and can affect a large number of domestic animals (the presence of RVF could also lead to an epidemic in humans who get exposed to diseased animals). The major factor behind epizootics is high population density for example the aquaculture industry is plagued with epizootics since in this industry a large population of fishes is kept in a confined area.

Q2. What is Meant by the Term “Herd Immunity”?

Ans: Herd immunity or population immunity is indirect protection from infectious disease. This usually happens due to either vaccination against the disease or the development of immunity in the population due to the previous infection. This means that even if people are not vaccinated or if the vaccine did not trigger immunity in some people, they could still be protected from the infection as the immune people around them act as buffers between the infection and susceptible people. Once herd immunity is established for some time and the disease’s ability to spread is hampered, it is possible to eventually eliminate the disease. This is one of the ways smallpox ott eradicated from the world.