A pandemic (from Greek πᾶν pan "all" + δῆμος demos "people") is an epidemic of infectious disease that has spread through human populations across a large region; for instance multiple continents, or even worldwide. A widespread endemic disease that is stable in terms of how many people are getting sick from it is not a pandemic. Further, flu pandemics generally exclude recurrences of seasonal flu. Throughout history there have been a number of pandemics, such as smallpox and tuberculosis. More recent pandemics include the HIV pandemic and the H1N1 pandemics of 1918 and 2009.
Read more about Pandemic: Definition and Stages, Pandemics and Notable Epidemics Through History, Biological Warfare