Medically Reviewed by Jennifer Robinson, MD on April 18, 2022
4 min read

You probably know that COVID-19, the illness caused by the new coronavirus SARS-CoV-2, is a pandemic. But what’s the difference between a pandemic, an epidemic, and an outbreak? And when does a disease become a public health concern? Here are the basics of the spread of serious diseases and what you can do to protect yourself, your family, and your community.

Let’s start with the meanings of each word.

An outbreak is when an illness happens in unexpected high numbers. It may stay in one area or extend more widely. An outbreak can last days or years. Sometimes, experts consider a single case of a contagious disease to be an outbreak. This may be true if it’s an unknown disease, if it’s new to a community, or if it’s been absent from a population for a long time.

An epidemic is when an infectious disease spreads quickly to more people than experts would expect. It usually affects a larger area than an outbreak.

A pandemic is a disease outbreak that spreads across countries or continents. It affects more people and takes more lives than an epidemic. The World Health Organization (WHO) declared COVID-19 to be a pandemic when it became clear that the illness was severe and that it was spreading quickly over a wide area.

The number of lives lost in a pandemic depends on:

  • How many people are infected
  • How severe of an illness the virus causes (its virulence)
  • How vulnerable certain groups of people are
  • Prevention efforts and how effective they are

The WHO’s pandemic alert system ranges from Phase 1 (a low risk) to Phase 6 (a full pandemic):

  • Phase 1: A virus in animals has caused no known infections in humans.
  • Phase 2: An animal virus has caused infection in humans.
  • Phase 3: There are scattered cases or small clusters of disease in humans. If the illness is spreading from human to human, it’s not broad enough to cause community-level outbreaks.
  • Phase 4: The disease is spreading from person to person with confirmed outbreaks at the community level.
  • Phase 5: The disease is spreading between humans in more than one country of one of the WHO regions.
  • Phase 6: At least one more country, in a different region from Phase 5, has community-level outbreaks.

There’s no sure way to prevent the spread of disease during an outbreak, epidemic, or pandemic. It might take scientists a long time to make a vaccine. But it’s easier to make specific vaccines more quickly now than it was several years ago. Once a vaccine is ready, people and groups who are more likely to become ill will get it first.

In the meantime, you can take other steps to stay healthy:

  • Wash your hands often with soap and water. If that’s not an option, use an alcohol-based hand cleaner or gel sanitizer. Rub it on your hands until they’re dry.
  • Don't touch your mouth, nose, and eyes unless you've just washed your hands.
  • When you cough or sneeze, cover your mouth and nose with a tissue. Then throw the tissue in the trash. Wash your hands afterward.
  • Avoid crowded places. Stay home if you can.
  • Clean and disinfect household surfaces every day.

If you get sick:

  • Stay home and away from other people. If you want to talk to your doctor, call before you go to their office. But if you have severe symptoms like trouble breathing, call 911 or go to an emergency room right away.
  • Wear a face mask if you have to go out for medical care. Avoid public transportation, ride-hailing, and taxis.
  • Have only one person care for you, if possible.
  • Wash your hands often, and keep household surfaces clean and disinfected.

A pandemic causes economic and social problems because so many people are ill or can’t work.

Here are a few things you can do to help your family and your community before and during a pandemic:

  • Make an emergency contact list.
  • Find local aid organizations in case you need information, support, or health services.
  • Find out whether you can work from home.
  • Plan home learning activities in case school is closed.
  • Store extra water, food, medicine, and supplies.
  • Stay as healthy as you can by getting rest, managing stress, eating right, and exercising.
  • Help seniors and neighbors by sharing information and resources.

For more information on what to do in a pandemic, call the CDC Hotline at 800-CDC-INFO (800-232-4636) or go to

Many diseases are so common that we barely think about them. You might hear experts use one of these terms to describe them:

  • Sporadic means cases are rare and happen unevenly.
  • Endemic means a disease is constant and happens about as often as expected.
  • Hyperendemic means an illness is constant but people are getting sick at a higher rate.

Cases can also come in a cluster, a group of illnesses in a certain place and time.

The list of the deadliest pandemics in world history includes:

  • The Black Death. Experts think the plague, sparked by bacteria called Yersinia pestis, is to blame for the illness that tore through Europe in 1347-51. An estimated 25 million people died.
  • The influenza pandemic of 1918. At least 50 million people around the world died of flu during the outbreak of 1918-19. It’s often called the “Spanish flu,” not because the virus started there but because Spain was one of the first countries to announce cases.
  • Smallpox. The smallpox pandemic stretched over hundreds of years. Experts estimate that it killed as many as 300 million people in the 20th century alone. Thanks to widespread vaccine use, it was declared eradicated in 1980.
  • HIV and AIDS. The human immunodeficiency virus (HIV), acquired immunodeficiency syndrome (AIDS), and related illnesses have killed about 32 million people around the world.

Flu also killed millions of people worldwide in other pandemics:

  • 1957 (1.1 million)
  • 1968 (1 million)
  • 2009 (up to 575,000)