"Panˈdemik/: pan means "all"; demic (or demographic) means "people." It is well-named, because pandemic flu spreads easily throughout the world. Unlike seasonal flu, pandemics occur when a completely new or emerges. This sort of virus can emerge directly from animal reservoirs or be the result of a dramatic series of mutations -- so-called events -- in previously circulating viruses. In either case, the result is something mankind has never seen before: a that can spread easily from person to defenseless person, our immune systems never primed to launch any sort of defense."