According to the World Health Organization (WHO), “a pandemic is a worldwide spread of a new disease.” Before a disease is declared as a pandemic, it has to exceed a few other levels, according to the Centers for Disease Control and Prevention (CDC):
Sporadic:
When a disease occurs infrequently and irregularly.
Endemic:
A constant presence and/or usual prevalence of a disease or infection within a geographic area.
Epidemic:
A sudden increase in the number of cases of a disease — more than what's typically expected for the population in that area.
Pandemic:
An epidemic that has spread over several countries or continents, affecting a large number of people.