Pandemics occur when an epidemic spreads widely enough to encompass the world (or at least a very large portion of it). They could also occur when a new strain of virus emerges for which people have no previous exposure. Although medical advances and population characteristics have substantially improved the ability of epidemiologists and health care providers to cope with pandemics, they can still strike with unpredictable severity.
Think of the Spanish Flu, which emerged a century ago. It was possibly the most significant pandemic in human history and an epidemiological event still unparalleled in geographic reach, infectiousness, and virulence (severity). Spanish Flu spread rapidly throughout the world, killed with remarkable swiftness, and had strong staying power. All told, an estimated 50 million people worldwide died during its unusually long 18-month cycle.
Although influenza is not feared nearly as much today as it was 100 years ago, due to more effective medical and non-pharmaceutical interventions and better health care, non-seasonal influenza still poses high pandemic risk.
Pandemic influenzas can be characterized by rapid and frequent mutation, high human-to-human transmissibility, and variable virulence. Although prior exposure, especially early in life, can partially mitigate severity, the virus's high mutability has meant continued population vulnerability.
Indeed, the H1N1 virus, which caused Spanish Flu, has been a factor in several large-scale influenza outbreaks — most recently in 2009's Novel A/H1N1 Swine Flu pandemic. And H3N2, the fast-mutating culprit behind this year's seasonal flu epidemic, is causing the current flu season to quickly approach 2009's in population impact. Indeed, by mid-February 2018 — more than midway through the current Northern Hemisphere flu season — cases have been reported throughout the U.S. and in Puerto Rico, and more than 60 pediatric deaths have been recorded as of this writing.
