Pandemic- Definition, Features, Causes, Effects, Examples

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Pandemic Definition

An epidemic usually affecting a large proportion of the population, occurring over a wide geographic area such as a section of a nation, the entire nation, a continent, or the world is called a pandemic.

  • The word pandemic comes from the Greek pandemos meaning “pertaining to all people.” The Greek word pan means “all” and the Greek word demos means “people.”
  • It is the worldwide spread of a new disease.
  • Throughout history, there have been a number of pandemics, such as smallpox, tuberculosis, influenza, etc.
  • One of the most devastating pandemics was the Black Death, which killed over 75 million people in 1350.
  • The most recent pandemics include the HIV pandemic as well as the 1918,  2009 H1N1 pandemics and COVID-19 pandemics.
  • Besides humans, pandemics can also occur in important agricultural organisms (livestock, crop plants, fish, and tree species) or in other organisms.
Pandemic- definition, features, causes, effects, examples

Features of Pandemics

  • Pandemics are usually caused by a novel infectious agent, an infectious agent that is newly capable of spreading rapidly, or both.
  • The death toll in a pandemic is generally higher than that in an epidemic. It can also lead to more social disruption, economic loss, and general hardship.
  • Increased travel and mobility have increased the likelihood of new diseases spreading.
  • Antibiotic resistance increases the risk of future pandemics.
  • A disease or condition is not a pandemic merely because it is widespread or kills many people; it must also be infectious.
  • For instance, cancer is responsible for many deaths but is not considered a pandemic because the disease is not infectious or contagious.

Common Causes of Pandemics

  • Pandemic is usually caused by a new virus strain or subtype that becomes easily transmissible between humans.
  • Due to bacteria that become resistant to antibiotic treatment.
  • Sometimes, pandemics are caused simply by a new ability to spread rapidly, such as with the Black Death.
  • Pandemics arise when humans may have little or no immunity against a new virus. Often a new virus cannot spread between people, but if it changes, or mutates, it may start to spread easily. In this case, a pandemic can result.

Effect of Pandemics

Fatality from a pandemic depends upon:

  1. The number of people who become infected
  2. The severity of disease caused by the virus (its virulence)
  3. The vulnerability of affected populations
  4. The effectiveness of preventive steps

Examples of Pandemic Disease

The Influenza Pandemic

  • There are three groups of influenza viruses — A, B, and C. 
  • Each of these groups can cause illness in humans, but only group A viruses are associated with major epidemics or pandemics.
  • Influenza A viruses infect humans and other animals, notably birds and swine.
  • Influenza A viruses are continually undergoing evolutionary changes in their two major antigenic constituents – hemagglutinin (H) and neuraminidase (N). 
  • There are 16 H strains and 9 N strains of influenza A viruses currently recognized.
  • The viruses are named according to their H and N components, as well as the location where they were originally isolated.
  • The evolutionary changes in H or N (drift) may result in a sufficiently changed antigenic structure that immunity from prior infections may not protect all those who come in contact with it, contributing to annual epidemics. 
  • In addition, simultaneous infection with two different influenzas A viruses (in humans or other animals) may result in a reassortment of H and N components — antigenic shift — giving rise to a novel strain against which no one has immunity.  I
  • In this circumstance, pandemics of influenza may occur.

The World Health Organization (WHO) provides an influenza pandemic alert system, with a scale ranging from Phase 1 (low risk of a flu pandemic) to Phase 6 (a full-blown pandemic):

  • Phase 1: A virus in animals has caused no known infections in humans.
  • Phase 2: An animal flu virus has caused infection in humans.
  • Phase 3: Sporadic cases or small clusters of the disease occur in humans. Human-to-human transmission, if any, is insufficient to cause community-level outbreaks.
  • Phase 4: The risk for a pandemic is greatly increased but not certain.
  • Phase 5: Spread of disease between humans is occurring in more than one country of one WHO region.
  • Phase 6: Community-level outbreaks are in at least one additional country in a different WHO region from phase 5. A global pandemic is underway.


  7. Park, K. (n.d.). Park’s textbook of preventive and social medicine.
  8. Hennekens CH, Buring JE. Epidemiology in Medicine, Lippincott Williams & Wilkins, 1987.

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is doing his Ph.D. at the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal. He was awarded the DAAD Research Grant to conduct part of his Ph.D. research work for two years (2019-2021) at Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. Sagar is interested in research on actinobacteria, myxobacteria, and natural products. He is the Research Head of the Department of Natural Products, Kathmandu Research Institute for Biological Sciences (KRIBS), Lalitpur, Nepal. Sagar has more than ten years of experience in blogging, content writing, and SEO. Sagar was awarded the SfAM Communications Award 2015: Professional Communicator Category from the Society for Applied Microbiology (Now: Applied Microbiology International), Cambridge, United Kingdom (UK). Sagar is also the ASM Young Ambassador to Nepal for the American Society for Microbiology since 2023 onwards.

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