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  • While listening to the evening news you hear of an outbreak of

  • Ebola virus and the fact that the influenza virus is at pandemic levels

  • Both of these terms sound ominious, and may sound like they are referring to the same thing.

  • However to the scientists who study the rates of diseases in populations,

  • called epidemiologists, these terms have very different meanings.

  • Let’s use influenza, or the common flu, to explain the differences.

  • In this example we have a population of individuals,

  • each represented by a green triangle.

  • The population can be very large, such as an entire country, or relatively small, such as a group of villages.

  • Normally, each year, some of these people, indicated in red,

  • come down with the flu.

  • If the flu normally strikes a certain percentage of the population, and this year the rate of the flu is

  • what is expected, we say that the flu is endemic to that population.

  • Now let’s say that it is an above average flu season, and that more

  • people than normal are contracting the flu in our sample population.

  • The flu is then said to be an epidemic, meaning that it is occurring at a higher

  • rate, but is still isolated to the original population.

  • What happens if the disease starts to move between populations?

  • This is called an outbreak. An example would be an influx of the flu

  • from a population in Asia to a population in Europe.

  • Notice that it does not mean that the disease is occurring at a high rate

  • rather, it simply means that the disease has simply moved between populations.

  • Often what can happen after an outbreak is that the disease can become a pandemic.

  • In a pandemic, the disease starts to move between multiple populations

  • that are often geographically separated from one another.

  • Each the year the flu becomes a pandemic because it strikes populations of humans on almost every continent.

  • So let’s once again use the flu as an example.

  • If a new form of flu, endemic to areas of southeast Asia

  • starts to increase in frequency in those populations, then it is said to be an epidemic.

  • An outbreak occurs if the flu moves from Southeast Asia to southern Europe.

  • If it then moves to North America and Africa, it is said to be a pandemic.

  • Of course, the terms are all relative to the sizes of the populations being studied,

  • but it is important to note that these terms refer to how the disease is moving between populations.

While listening to the evening news you hear of an outbreak of

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