The World Health Organization has declared COVID-19 a pandemic. This is a landmark event.

As an epidemiologist listening to the steady stream of conversation around the coronavirus, I’m hearing newscasters and neighbors alike mixing up three important words my colleagues and I use in our work every day: outbreak, epidemic and pandemic.

Simply put, the difference between these three scenarios of disease spread is a matter of scale.

Outbreak - Small, but unusual.
By tracking diseases over time and geography, epidemiologists learn to predict how many cases of an illness should normally happen within a defined period of time, place and population. An outbreak is a noticeable, often small, increase over the expected number of cases.

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