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):