What Is a Pandemic?

Medically Reviewed by Dan Brennan, MD on December 24, 2022

A pandemic is the spread of a new disease around the world. Health experts and scientists agree that it means a surge in illness over a large area. But there’s some dispute about other ways to define a pandemic, like whether the disease is new, whether it’s spread in a short time, and how severe it is.

Before calling a disease a pandemic, experts consider these things:

  • How far has it spread? A pandemic is when an illness spans many countries or continents, or over a wide area.
  • How many people has it affected? A pandemic involves a large number of people.
  • Is it a new disease? A new illness or strain of disease often causes a pandemic. Since our bodies have little or no immunity (protection) against it, it spreads quickly.

Just like with the current pandemic, COVID-19, there are steps people can take to help get it under control.

Scientists who study health events and how they affect people (epidemiologists) are careful about whether they call a disease a pandemic because it greatly impacts daily life for millions of people, society, businesses, and politics.

During a pandemic, federal and local governments and health officials can take more serious action to fight and control a disease. They may send resources like life-saving medicine and medical supplies from a national stockpile to communities that need them the most. In the U.S., the FDA may let doctors use drugs, vaccines, and devices that haven’t been approved for the pandemic illness. The government can also take measures like checking people traveling into the U.S. or between states, separating them, or limiting their movement to protect public health.

A key goal is what’s called "flattening the curve." When experts plot illnesses on a graph, it’s better for public health if there’s a longer, flatter curve (with illnesses spread out over time) than a shorter, taller one (with many people sick at once).

Flattening the curve involves everyone taking steps to limit the spread. This may include things like getting vaccinated, staying home, avoiding large crowds (social distancing), washing your hands often, cleaning your home, and limiting travel.

You may also hear the term “herd immunity.” That is when a large enough portion of a population becomes immune to an infectious disease that it helps slow the spread. This can happen when a significant portion of the population is vaccinated or carry the antibodies after having had been ill.

Pandemics are rare, but they do happen. Things you can do during a pandemic to help protect yourself are:

  • Avoid close contact. Stay away from people who are sick, or if you're sick.
  • Practice good hygiene. When you sneeze or cough, cover your mouth and nose with a tissue or the crook of your elbow, not your hand. Wash your hands often with soap and clean water. Don't touch your face.
  • Wear a face mask when you’re out. This helps because it might be hard to stay at least 6 feet away from other people at all times.
  • Stay healthy. Be sure to get enough sleep and regular exercise. Manage stress, and eat healthy food.

When there’s no pandemic -- which is most of the time -- it’s a good idea to prepare, just like you would for any other emergency. For instance, stock up on essentials (without hoarding them) and have a plan for how you’d handle daily life in a crisis.

The word "pandemic" has been around since the 1600s. Back then, people thought of it in the same way as an "epidemic”: a sudden spike in disease. By the 1800s, scientists had started to understand disease patterns and how they spread around the world.

The cholera pandemic of 1831 was the first time the public tracked a global illness in the press, day by day. By the time of 1889's influenza (flu) pandemic, most knew the word to mean the worldwide spread of a disease that sickens many people.

Global pandemics over the past century include:

COVID-19 (2019). In March 2020, the World Health Organization (WHO) announced that COVID-19, or coronavirus disease 2019, had become a pandemic.

H1N1 (2009). Doctors and scientists discovered the illness, also called swine flu, in the United States. Between 151,000 and 575,000 people around the world died of the virus during its first year.

HIV (1981). HIV was identified in the early 1980s. The pandemic form has infected more than 60 million people and killed 25 million.

Influenza (1918). The flu of 1918-19 killed more than 50 million people and infected an estimated one-third of the world's population. It’s sometimes called “Spanish flu,” not because it started in Spain but because Spain was one of the first countries to announce cases.

Some people use the word "pandemic" casually to describe things that involve a lot of people, whether they’re related to health or not. These might include smoking, obesity, diabetes, or traffic accidents. Because they're not contagious diseases, they're not true pandemics.

Show Sources


World Health Organization: "What is a pandemic?" "WHO Director-General's opening remarks at the media briefing on COVID-19 -- 11 March 2020," "The elusive definition of pandemic influenza."

The Journal of Infectious Diseases: "What Is a Pandemic?"

CDC: "Principles of Epidemiology in Public Health Practice," "2009 H1N1 Pandemic (H1N1pdm09 virus)," "Coughing & Sneezing," "Regulations and Laws That May Apply During a Pandemic," " Legal Authorities for Isolation and Quarantine,” “Partner Key Messages on the 1918 Influenza Pandemic Commemoration,” About Cloth Face Coverings.”

National Institutes of Health: "What Is Epidemiology?"

Cold Spring Harbor Perspectives in Medicine: "Origins of HIV and the AIDS Pandemic."

Emerging Infectious Diseases: "1918 Influenza: the Mother of All Pandemics."

Association for Professionals in Infection Control and Epidemiology: "Outbreaks, epidemics and pandemics -- what you need to know."

Department of Homeland Security: "Pandemic."

Public Health Emergency: "Strategic National Stockpile."

Michigan Health: "Flattening the Curve for COVID-19: What Does It Mean and How Can You Help?"

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