2009-06-05 / Columnists

PHC Health Talk

By Daniel G. Canario, M.D. Chief, Infectious Disease Division, Peninsula Hospital Center

Daniel G. Canario, M.D. Daniel G. Canario, M.D. With the advent of swine flu, the public has been bombarded with a wealth of confusing information and with public concern about this illness spreading, understanding ht terminology is essential.

Perhaps no two words are more confused and confusing than "pandemic" and epidemic." Are they two different words for the same thing? Or do they have very different definitions, and merely sound alike?

Epidemic is from the Greek epi, which means "upon" and demos, which means "people." In current terminology, an epidemic is a disease which appears as new cases in a specific human population, during a specific period of time, at a rate which significantly exceeds what is expected based on recent experience.

For your information, the number of new cases in the population during a specified period of time is called the "incidence rate."

A pandemic, on the other hand comes from the Greek pan, which means "all" and demos, which as we have just learned, is "people." Thus, a pandemic is actually a very specific type of epidemic; one that spreads across either a large region (a country or a continent) or worldwide.

The best way to think about it is that when an epidemic is out of control, it is termed a pandemic. But this must be further qualified because a pandemic can also simply be an epidemic that is not specific to a city or a small geographic region but spans a larger geographical area.

Additionally, an epidemic may be local to a small region but the number of people affected may be very large compared to what is the "norm." In this instance too, such an outbreak is also termed a pandemic, even if it is relatively localized.

For example, take a disease that has an expected rate of infection of 20 percent.

When 45 percent of the population of a city or state or other geographic area is infected, that would be an epidemic. When 80 percent of the population becomes infected, it has reached pandemic proportions.

As a side note, both epidemics and pandemics in a medical sense always refer to the spread of infectious diseases.

There is a subtle distinction of when an epidemic becomes a pandemic. Let's say that 20 percent of everyone in the US came down with the flu; that would still be an epidemic because the rate is only slightly greater than the percentage that would normally be affected. But let's say you look at Africa, where a relatively small number of people are spread over a large geographic area. And let's say that 95 percent of this small population becomes infected, than you have a pandemic.

So, both factors must exist: an incidence much higher than epidemic proportions and one that is spread over a large geographic area.

Just a little bit of trivia… the last word in diction ary publishers, Merriam- Webster, reports that "pandemic" is one of the top ten most frequently looked-up words in its online dictionary.

The World Health Organization (WHO) monitors flu cases throughout the world. The agency 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.

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