Read More: Scientific American
This year the world will mark the 100th anniversary of one of the most devastating infectious disease events in recorded history: the 1918 influenza pandemic, which caused an estimated 50 million to 100 million deaths worldwide. There were several reasons for the awful toll. First, most people likely had no preexisting immune protection to the brand-new strain that had emerged. Second, this particular virus may have been unusually lethal. Third, crowding and poor sanitation allowed for rampant disease transmission, especially in regions where access to health care was limited. And finally, antiviral drugs and flu vaccines were still decades in the future. Over the past century we have made substantial advances in all these areas. But we are still unprepared for the inevitable appearance of a virus like the one that struck a century ago. Even an ordinary seasonal flu epidemic will still kill some 12,000 to 56,000 people every year in the U.S. alone. That is because seasonal viruses continually evolve, and although we update our vaccines frequently, they may be only 40 to 60 percent effective. Moreover, seasonal vaccines may provide little or no protection against pandemic flu. Pandemic viruses typically arise from a process referred to as an antigenic shift, in which the new virus acquires, usually from animal influenza viruses, one or more genes that are entirely novel (as seems to have happened in 1918, when all eight pandemic virus genes were novel). In the years since 1918, three influenza pandemics associated with antigenic shifts occurred: in 1957, 1968 and 2009. In each of these instances, however, the new viruses emerged via the mixing of animal influenza virus genes with those of the 1918-descended viruses already circulating in the human population, which meant that many people were at least partially immune. That, plus lower viral pathogenicity and improvements in public health infrastructure and medical treatment, is what probably led to less catastrophic pandemics.