A pandemic typically refers to a situation where an infectious disease becomes newly widespread within a large geographical area and/or globally. In popular culture, the term pandemic has become nearly synonymous with numerous cases of life-threatening illness; however, the severity of the resulting disease is not an indicator of pandemic occurrence. Key to determining whether the disease has reached pandemic proportions is the global location of infected individuals. Large-scale spread of disease is made possible by limited population immunity to the newly formed and/or newly released pathogen. Pervasive illness can result from an overall increase in infection rates from intrinsic behavior of the disease-causing biological agent, as well as its accidental or intentional (bioterrorist) release.

The World Health Organization (WHO) divides pandemics into six stages, ranging ...