An outbreak of an infectious disease is generally considered to be a local phenomenon, while an epidemic occurs when new cases of a certain disease during a given period substantially exceed what is expected based on recent experience. A pandemic is said to occur when an epidemic of infectious disease has spread across a large region; for instance multiple continents, or even worldwide.
Throughout history there have been a number of pandemics, such as smallpox & tuberculosis. More recent pandemics include the HIV pandemic and the H1N1 pandemics of 1918 & 2009. Most pandemics of today have involved the flu.
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