What Is a Pandemic?

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

Medically Reviewed by Ann Edmundson, MD, PhD

"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."

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.

The world is closely watching a virus known as avian influenza H5N1, or "bird flu." Don't confuse it with pandemic flu. It isn't one. At least, it isn't one yet.

At this point it's known that people have caught the virus from sick poultry, and that the virus is very deadly to people who are infected. Scientists worry that at some point the H5N1 virus will mutate into a form that can pass from human to human, which it cannot do at present.

"If it adapts to a strain that's contagious among humans it will no longer be a bird virus. It will become a human influenza virus," Epstein tells WebMD.

Then, if this hypothetical strain is able to pass easily between people, it may become a pandemic flu.

"It's impossible to predict whether this virus will mutate enough to be easily passable from human to human," Pearson tells WebMD.

Another flu pandemic is almost a certainty. But an entirely different virus may cause the next pandemic. It will not necessarily develop from H5N1.

The three pandemics of the 20th century were caused by what are known as "type A" flu viruses. It's possible that a type A virus that's in circulation among humans today may change into a new strain that's very contagious. Then we might have a pandemic.

The CDC keeps track of the influenza strains that circulate widely in the U.S. each year. In the 2004-2005 flu season, the dominant strains were influenza type A (H3N2) and influenza type B viruses. A version of the virus responsible for the 1918 pandemic, type A (H1N1), also circulated.

The World Health Organization (WHO) constantly monitors flu cases throughout the world, relying on information from a wide network of sources, including government health agencies, university scientists, and international aid organizations.

WHO has developed a system of identifying where the world stands with regard to pandemic flu. The system has six phases:

  • Phase 1 -- No new influenza virus has been found in people or animals.
  • Phase 2 -- New virus has appeared in animals, but no human cases.
  • Phase 3 -- A new strain of animal influenza virus infects humans, but there have not been human-to-human infections.
  • Phase 4 -- The new virus passes from person to person, but transmission is limited and confined to a certain location.
  • Phase 5 -- There is frequent transmission of the virus between people in a particular place, but it hasn't spread to the rest of the world.
  • Phase 6 -- Pandemic. The virus is widespread worldwide.

We are currently in phase 3, which marks the beginning of the "pandemic alert period," because of what has been developing with avian influenza virus H5N1.

It's possible that the H5N1 will turn into a human influenza virus. But if it does, it may never be contagious enough to spark a pandemic. Or a virulent new strain may be contained before it can spread far.

The world waits, watches, and tries to prepare.

Show Sources

SOURCES: Christine Pearson, spokeswoman, CDC. Dan Epstein, spokesman, Pan American Health Organization. CDC. WHO. Kiple, K., ed. The Cambridge World History of Human Disease, Cambridge University Press, 1993. News release, Merriam-Webster.

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