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After searching a number of sources, we located an article in the CDC's Emerging Infectious Diseases journal that describes the complicated statistical process used to determine influenza epidemics in the U.S., France, and Australia from 1972 to 1997. We're not statisticians, but it seems an epidemic is defined by how many deaths are caused by a disease during an outbreak. If the mortality rate is above average, the outbreak is an epidemic. This may be easier to determine with seasonal illnesses such as the flu, when lots of data is available to estimate future mortality rates. This annual data makes it more obvious when deaths are increasing rapidly or spreading quickly. On average, 36,000 people die from
influenza in the U.S. each year. The CDC's Morbidity and Mortality Weekly Report for December 13 notes that influenza deaths this flu season have not reached the statistical epidemic threshold. However, the media and even the CDC uses the term "epidemic" casually. CNN quotes the CDC's Dr. Julie Gerberding as saying: "There is no firm dividing line between what is an epidemic and what is not an epidemic, but I think, when you look at a map that shows widespread influenza activity in 36 states, that we regard it -- from a common-sense perspective -- as an epidemic." A pandemic has a rather loose definition too. WHO states that influenza pandemics occur "when a new influenza virus appears against which the human population has no immunity, resulting in several, simultaneous epidemics worldwide with enormous numbers of deaths and illness." It seems to be a matter of scale. Pandemics are several, serious epidemics that spread quickly and affect large numbers of people. In early 2003, a new disease called severe acute respiratory syndrome (SARS) touched more than two dozen countries, especially in Asia. But this was neither a pandemic nor epidemic. Only 8,098 people were infected with SARS, and 774 died worldwide before the outbreak was contained. In contrast, WHO aptly calls AIDS an epidemic, as the disease killed at least 2.5 million people in 2003 alone. In 1918, a major influenza pandemic spread rapidly and killed upwards of 30 million people worldwide.
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