Classifying Infectious Diseases as Pandemics
The word pandemic comes to us from the Greeks. Pan- means "all" and demos means "the people," so a pandemic is an infectious disease outbreak with the potential to affect all people over a large geographic region, or even worldwide. In epidemiology, a pandemic is distinguished from an endemic (a stable but communicable disease like chickenpox in North America) and from an epidemic (an infectious disease spreading more than expected, but restricted to a smaller geographic region.)
Classification of a pandemic
The World Health Organization (WHO) uses a 6-phase pandemic classification index. Risk is low in phase 1 (animal infections only) and phase 2 (limited animal-to-human transmission) of an infectious disease outbreak. At phase 3 there may also be limited human-to-human transmission. Phase 4 sees human-to-human transmission in a "community-level outbreak." Phase 5 sees the disease spread to at least two countries within a single WHO administrative region. Only at phase 6, which involves spread to a second geographic region, is a pandemic officially declared.
A misunderstanding of what constitutes a pandemic has led to criticism over how the WHO handled the 2009 H1N1 global pandemic. The American Centers for Disease Control and Prevention (CDC) have developed a 5-category Pandemic Severity Index (PSI) that responds to the public's desire for information on severity of individual cases of a disease. All infectious disease outbreaks measured on the PSI are pandemics, however the scale puts into perspective the destructive potential of a disease. It allows comparison of a new pandemic, like the 2009 swine flu, with past ones such as the much more deadly 1918 Spanish flu.
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