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What Is a Pandemic?

Confused about what a pandemic is versus an epidemic? WebMD explains.
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WebMD Feature

"Pandemic flu" has been one of the buzzwords of late 2005. But how does the phrase that's on everyone's lips differ from "epidemic," that other well-worn disease term?

Apparently, a lot of people aren't sure. Merriam-Webster reports that "pandemic" is the seventh most frequently looked-up word in its online dictionary this year. The definition: "occurring over a wide geographic area and affecting an exceptionally high proportion of the population."

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This is almost the same as the dictionary definition for "epidemic," and it doesn't explain much when it comes to influenza -- a.k.a. the flu.

An epidemic of influenza is different from the dreaded pandemic that scientists and world health officials fear is nigh. We might see an epidemic of seasonal influenza during any given year. In fact, we just had one.

Flu reached epidemic levels in the U.S. for 10 weeks in a row during the 2004-2005 season. Records kept by the CDC show that during the week ending March 5, 2005, 8.9% of all deaths reported in 122 U.S. cities were due to influenza and pneumonia (a common complication of the flu).

The CDC's definition of a flu epidemic relates to the percentage of deaths in a given week caused by influenza and pneumonia. The "epidemic threshold" is a certain percentage above what is considered normal for that period. The normal level, or baseline, is statistically determined based on data from past flu seasons.

Christine Pearson, a spokeswoman for the CDC, cautions that the definition of an influenza epidemic doesn't apply to other diseases.

Seasonal flu epidemics may sicken millions, but those who die are typically a small number of the elderly, very young children, and people with weak immune systems. That's not the case during the worst influenza pandemics.

There are two main features of an influenza pandemic. First, the virus is a new strain that has never infected people before. Second, it's on a global scale. Sometimes it's also unusually deadly.

"A pandemic is basically a global epidemic -- an epidemic that spreads to more than one continent," says Dan Epstein, a spokesman for the Pan American Health Organization, a regional office of the World Health Organization.

Influenza pandemics have struck about three times every century since the 1500s, or roughly every 10-50 years. There was one in 1957-1958 and one in 1968-1969. The most infamous pandemic flu of the 20th century, however, was that of 1918-1919. An estimated 40 million people died in less than a year, and what made it so different from seasonal flu epidemics is that it killed primarily young people, those aged 20-45.

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