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.