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COVID-19, the disease caused by novel coronavirus SARS-CoV-2, reached a new level March 11, 2020, when the World Health Organization (WHO) declared it a pandemic. It became the first pandemic in history caused by a coronavirus.

The WHO defines a pandemic as “the worldwide spread of a new disease.” People do not have immunity to the new disease, which can cause high numbers of illnesses and deaths. Pandemics often cause disruptions in society.

Between December 2019, when COVID-19 emerged in China, and March 11, 2020, the disease spread to more than 100 countries. It has caused millions of illnesses and hundreds of thousands of deaths worldwide.

Before COVID-19, the most recent global pandemic was the H1N1 flu in 2009.

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The Difference Between Epidemics and Pandemics

Several different terms describe health conditions within a local or widespread population, according to the Journal of the American Medical Association:

  • Endemics: An endemic occurs when a health condition spreads at a steady, expected rate within a population. The population can be as small as a town or as large as a continent, but the condition affects a predictable level of people. The malaria levels in Africa can be classified as an endemic.
  • Outbreaks: When there is a sudden increase in a health condition in a small area, it becomes an outbreak. Outbreaks either can come from the rapid growth of an endemic or a condition that appears somewhere for the first time. An outbreak of cholera began in Haiti after a 2010 earthquake.
  • Epidemics: When an outbreak spreads over a larger global area, it becomes an epidemic. For example, the Zika virus began in Brazil in 2014 but spread to many other areas of Latin America and the Caribbean.
  • Pandemics: When an epidemic spreads worldwide, it becomes a pandemic. A disease does not have to cause a certain number of illnesses to become a pandemic. The WHO declares an influenza pandemic if a certain flu virus reaches more than one global region. COVID-19 reached all six of the WHO’s defined global regions.

Pandemics Through History

Pandemics have occurred throughout history. The first recorded pandemic occurred in 430 B.C., during the Peloponnesian War fought between Greek city-states. Nearly two dozen pandemics have followed in the centuries since.


Notable pandemics include:

  • Plague of Justinian in the sixth century, which caused at least 30 million deaths and may have led to the fall of the Roman Empire
  • The Black Death, or bubonic plague, which caused up to 200 million deaths in Europe in the Middle Ages
  • Smallpox, which killed 56 million people from 1520 onward, including 90 percent of the Native American population
  • Spanish Flu, which caused between 40 million and 50 million deaths in 1918 and 1919
  • HIV/AIDS, which has caused between 25 million and 35 million deaths worldwide since 1981
  • H1N1 flu, which caused up to 200,000 deaths in 2009 and 2010
  • Sudden Acute Respiratory Syndrome (SARS), which caused more than 700 deaths in 2003

Pandemic Response in Recent Years

In recent years, the WHO and CDC increased planning on how to prepare for flu pandemics. The response came after avian flu spread from poultry to humans in 1997 in Hong Kong, killing six people.

The Centers for Disease Control and Prevention and WHO both developed strategies on how to control the spread of flu pandemics. The CDC guidelines aim to help with national, statewide, and local planning and response. The WHO protocols allow the WHO and its partners to help nations when a pandemic flu emerges, helping to control the spread.

The WHO says “detection, investigation, and reporting of the first cases must happen quickly” to contain a pandemic.

National Pandemic Strategy . Centers for Disease Control and Prevention. What Is a Pandemic? . Journal of the American Medical Association. Pandemics That Changed History . History Channel. Pandemic Influenza Preparedness and Response: A WHO Guidance Document, The WHO Pandemic Phases . World Health Organization. A Visual History of Pandemics . World Economic Forum. WHO Director-General’s Opening Remarks at the Media Briefing on COVID-19 – 11 March 2020 . World Health Organization. WHO Interim Protocol: Rapid Operations to Contain the Initial Emergence of Pandemic Influenza . World Health Organization.

About Infectious Diseases

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