A pandemic is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. It is a disease outbreak that spans several countries and affects a large number of people. Pandemics are most often caused by viruses, like Coronavirus Disease 2019 (COVID-19), which can easily spread from person to person. A pandemic is not the same as an epidemic. In an epidemic, many more cases of a health condition occur than would normally develop in a community or region, but the condition does not spread further.

The World Health Organization (WHO) has defined a pandemic as "the global spread of a pathogen or variant that infects human populations with limited or no immunity through sustained and high transmissibility from person to person, overwhelming health systems with severe morbidity and high mortality, and causing social and economic disruptions, all of which require effective national and global collaboration and action".

Pandemics cause economic and social problems because so many people are ill or can’t work. To prepare for a pandemic, it is important to make an emergency contact list, find local aid organizations, 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, and help seniors and neighbors by sharing information and resources.

In the case of influenza, which is the disease that poses the greatest pandemic threat to humans, WHO has organized a pandemic preparedness plan that consists of six phases of pandemic alert. The phases range from the lowest level of pandemic alert, which indicates that an influenza virus, either newly emerged or previously existing, is circulating among animals, to the highest level of pandemic alert, which indicates that a pandemic is occurring.