“Viruses used to spread at the speed of a steamboat; now they can spread at the speed of a jet.

(Infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security.)

Sometimes, an epidemic stays contained to a specific area—but when it extends into other countries or continents, and epidemic turns into a full-blown pandemic. That was the case in 2009 when the WHO declared the swine flu (caused by the H1N1 flu virus) a "Public Health Emergency of International Concern"—aka a pandemic.

Level of Disease;

  • 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. (Hyperendemic is a situation in which there are persistent, high levels of disease occurrence.)
  • 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.

source: Centers for Disease Control and Prevention (CDC), Harvard University, Dept. of the History of Science