What Is a Pandemic and How Does It Affect Us?

Confused About What's a Pandemic vs. an Epidemic? WebMD Explains

Medically Reviewed by Jabeen Begum, MD on November 06, 2023
Written by Kendall K. Morgan
7 min read

An outbreak of infection is considered a pandemic when it grows very fast. It starts out slowly at first and then takes off. Sometimes, people call this type of growth exponential.

For an outbreak to be a pandemic, it also has to affect lots of people in many countries or even continents. It spreads quickly and easily around the world all at once.

Pandemics usually happen when a virus such as the coronavirus that causes COVID-19 starts infecting people for the first time. New viruses can spread and infect you more easily than those that have been around before. That's because you don't have any immunity against them. You usually won't be able to get a vaccine or other medicines to protect you against a new virus either.

At first, doctors might not even know the identity of the new virus, how it spreads, or the best way to slow it down. Any type of virus may cause a pandemic, but some are more likely to do so than others. While the COVID-19 pandemic was caused by a coronavirus, many other pandemics in the past were caused by new strains of influenza or other viruses.

In a pandemic, new viruses infect lots of people and make them sick. But how sick you or others will get depends on the virus. It also will depend on other health conditions you have and how the new virus interacts with those. So, some pandemics are deadlier than others. The key factor in a pandemic is that an outbreak of infection is both fast-moving and widespread.

Pandemic vs. epidemic

An epidemic is when a virus spreads more than expected in a certain place. For example, the flu (influenza virus) spreads each winter in the U.S., but some years, it infects more people and causes more sickness and deaths than usual.

Other classic examples of viruses that have caused epidemics include:

  • Yellow fever
  • Smallpox
  • Measles
  • Polio

An epidemic may turn into a pandemic if it starts spreading even faster and in more places. An epidemic can still affect lots of people and cause many deaths. The difference is that it's affecting people only in a certain area, not globally. A pandemic is an epidemic that's happening around the world.

Sometimes, doctors also talk about epidemics of other health conditions, such as an epidemic of obesity, diabetes, or substance abuse. That means they're big health problems affecting lots of people in a certain place and time.

There's no way to know what the cause of the next pandemic will be. In the past, experts in infectious diseases and public health mainly tracked influenza viruses. That's because influenza viruses have often caused major pandemics in the past. They change all the time and spread each year, too. So, the U.S. and other places keep an eye on them so they can respond quickly if seasonal influenza or a new influenza virus shows signs it might trigger another pandemic.

Before the COVID-19 pandemic, most people didn't expect that a coronavirus would cause the next pandemic. Most coronaviruses don't make people very sick. But there had been other examples of coronaviruses that caused outbreaks, including severe acute respiratory syndrome (SARS) in 2002. COVID-19 in 2020 still came as a surprise.

The World Health Organization (WHO) constantly monitors viruses around the world with help from government health agencies, scientists, and international aid organizations. WHO has developed a system to identify where the world stands with regard to influenza and other viruses. The system has six phases:

  • Phase 1. No new virus has been found in people or animals.
  • Phase 2. A new virus is in animals, but there aren't any human cases.
  • Phase 3. A new strain of an animal virus infects humans. There may be limited spread of the virus from one person to the next in a certain place, but the virus isn't causing a community-wide outbreak.
  • Phase 4. The new virus starts spreading among people to cause an outbreak in a community, but its spread is still very limited.
  • Phase 5. The virus is spreading among people a lot in at least two countries, but it hasn't made it to the rest of the world. At this stage, the outbreak is an epidemic.
  • Phase 6. The virus is widespread worldwide. At this stage, the outbreak is a pandemic.

Another pandemic is almost sure to happen. But an entirely different virus may cause it. Some viruses are more likely to cause a pandemic than others because of how fast they change or how easily they spread. 

Some viruses that infectious disease experts are watching closely for their potential to cause an epidemic or pandemic include:

  • Coronaviruses
  • Zika virus
  • Influenza
  • Respiratory syncytial virus (RSV)
  • Viral hemorrhagic fevers, such as Ebola, Nipah, Marburg, and Lassa fever

WHO and others are also looking out for “Disease X.” That means they have to watch for a new virus and disease that they didn't expect. They especially need to watch for any new virus that emerges fast and spreads quickly to make people sick.

People first used the word “pandemic” in 1666. Some notable pandemics over time include:

  • Bubonic plague. It's also called the Black Death or the Plague. It happened in Europe, Asia, and Africa from 1346 to 1353 and is considered the deadliest and worst pandemic in history. The disease spread to people from fleas on rats. About 200 million people died.
  • Cholera. Cholera has caused several pandemics dating back to the 1800s. Cholera spreads through contaminated water or food. The disease has killed millions of people, with more than a billion now at risk around the world.
  • Spanish flu. The 1918 flu pandemic started in Kansas from an H1N1 influenza strain that came from birds. It killed so many young people that it dropped the average life expectancy to just 12 years.
  • Asian flu. Starting in China in 1957, an H2N2 influenza virus caused this pandemic. It also came from a bird strain and spread to the U.S., England, and Scotland fast. By 1958, it had killed more than a million people.
  • Hong Kong flu. An H3N2 influenza virus caused this outbreak, starting in Hong Kong in 1968. It quickly spread to the U.S. and other places, killing up to 4 million people.
  • HIV/AIDS. The human immunodeficiency virus (HIV) has killed 36 million people and continues to infect millions around the world. While there's no cure for HIV, medicines can now control the infection and keep it from spreading.
  • SARS. This pandemic was caused by a coronavirus related to the one that causes COVID-19. It started in 2002 in China and spread to 29 countries, killing 774 people. The pandemic ended in 2003.
  • Swine flu. In 2009, swine flu started spreading to infect about 1 in 10 people. The H1N1 influenza virus that caused it started in Mexico and mostly affected teens and young adults.
  • MERS. Middle East respiratory syndrome coronavirus, which emerged in 2012, spreads to people from camels. Since 2012, 27 countries have reported MERS cases, with a combined toll of more than 850 deaths.
  • Ebola. This hemorrhagic fever arose first in Guinea in 2013. Ebola is now a rare but very deadly disease, which can spread to people from other animals or people.
  • COVID-19. The coronavirus that causes COVID-19 showed up first in China in late 2019. It quickly spread to other countries. WHO declared it a pandemic in March 2020. Before the pandemic emergency ended 3 years later, the virus had killed more than 2 million people.

Even though you can't know when or what the next pandemic will be, you can still take steps to prepare and protect yourself. Some steps you can take include:

  • Know how infections spread. If you know how viruses or other infections can spread from one person to the next, you can take steps to make that less likely. 
  • Have supplies. It's a good idea to keep your home stocked with food and other supplies that could last you for days or weeks in case of emergency, including a future pandemic.
  • Make a plan. Talk with your family about what to do if an outbreak of infection or pandemic happens.
  • Review your policies. Make sure you have important documents in a safe spot and that you know how your health insurance works.

In the event of a public health emergency or pandemic, check your national, state, or local health departments for updates. Some steps you can take to keep yourself well in general include:

  • Staying up to date on your vaccines
  • Staying home when you're sick
  • Washing your hands often and/or using hand sanitizer
  • Keeping surfaces clean in your home
  • Keeping your distance from other people
  • Wearing a mask over your nose and mouth when you are out in public or around lots of people

Remember that not all viruses are the same. Some can make you more sick than others. Viruses can spread in different ways, too. In the event of a future epidemic or pandemic affecting the place where you live, follow the latest advice to protect your own health and that of your community.