Remember the 2009 swine flu? That was a pandemic — an illness that swept over much of the globe.
People fear pandemics, and with good reason. The black plague that ran through Asia and Europe in the 1300s is believed to have killed as much as half of Europe's population, between 75 and 100 million folks. The word comes from ancient Greek — pan (meaning "all") and demos (meaning "people"), or simply put — all the people. Of course, people across the globe suffer from the seasonal flu, and many die from it. But it isn't considered a pandemic because it is generally harmless to the healthy.