Epidemic Definition
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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FAQs on Epidemic
1. What is the precise definition of an epidemic?
An epidemic is defined as a sudden and significant increase in the number of cases of a particular disease, far exceeding what is normally expected within a specific community or geographical region. It signifies a rapid spread of the disease that is not contained and is growing.
2. What is the difference between an epidemic, a pandemic, and an endemic disease?
These terms describe the scale and prevalence of a disease. The key differences are:
- Endemic: Refers to a disease that is constantly present in a particular population or region at a predictable, low-to-moderate level. Malaria in certain parts of Africa is an example.
- Epidemic: A sudden outbreak of a disease that spreads rapidly and affects an unusually large number of people within a community or region.
- Pandemic: An epidemic that has spread across multiple countries or continents, affecting a large portion of the global population. COVID-19 is a recent example of a pandemic.
3. How does a disease outbreak become an epidemic?
An outbreak is a localised spike in disease cases, often the starting point of an epidemic. It officially becomes an epidemic when control measures fail to contain the outbreak and the disease begins to spread rapidly and widely through the population, substantially exceeding the normal incidence rate for that area.
4. What are some clear examples of epidemic diseases from history?
Throughout history, several diseases have caused major epidemics. Notable examples include:
- Cholera: Major epidemics occurred in the 19th century, often linked to contaminated water sources in densely populated cities.
- Ebola Virus Disease: The 2014-2016 outbreak in West Africa was a significant modern epidemic.
- Influenza (Flu): Different strains of influenza cause seasonal epidemics annually, with the potential to become pandemics.
5. What are the two main types of epidemics based on their spread?
Epidemics are generally classified into two types based on how they are transmitted:
- Common-Source Epidemic: This occurs when many people are exposed to a single infectious source, such as contaminated food or water. The number of cases rises rapidly and then falls once the source is removed.
- Propagated Epidemic: This type spreads from person to person. The number of cases rises and falls more slowly as it moves through the population, with each infected individual becoming a source for new infections.
6. Why is herd immunity so important for stopping an epidemic?
Herd immunity, or population immunity, is crucial because it protects the entire community, not just those who are immune. When a high percentage of the population is immune (through vaccination or prior infection), the chains of disease transmission are broken. This makes it difficult for the pathogen to find susceptible hosts, which slows the spread of the epidemic and protects vulnerable individuals who cannot be vaccinated, eventually causing the epidemic to die out.
7. Can a non-infectious condition like obesity be considered an epidemic?
Yes, the term 'epidemic' is now commonly used in public health to describe a rapid increase in any health-related issue, not just infectious diseases. When conditions like obesity, diabetes, or opioid addiction show a sharp rise in prevalence far beyond expected rates, they are often referred to as epidemics to highlight the urgency and scale of the public health crisis.
8. How do epidemiologists identify the source of an epidemic?
Identifying the source is a critical process known as an epidemiological investigation. It involves several key steps: confirming the diagnosis and existence of an outbreak, creating a specific case definition, performing contact tracing to map the disease's spread, collecting data on affected individuals, and using laboratory analysis to identify the pathogen. By analysing this data, scientists can pinpoint the common source or the initial case (patient zero) that triggered the epidemic.
9. What is an epizootic, and how does it relate to human epidemics?
An epizootic is the animal equivalent of a human epidemic—it's a disease event that affects an unusually high number of animals in a specific population and area. Epizootics are very important to human health because many human epidemics are caused by zoonotic diseases, which are infections that jump from animals to humans. For example, an epizootic of avian influenza ('bird flu') in poultry can lead to a human epidemic if the virus adapts to spread among people.
