Although the swine flu outbreak of 2009 is still in full swing, this global influenza epidemic, the fourth in 100 years, is already teaching scientists valuable lessons about pandemics past, those that might have been and those that still might be. Evidence accumulated this summer indicates that the novel H1N1 swine flu virus was not entirely new to all human immune systems. Some researchers have even come to see the current outbreak as a flare-up in an ongoing pandemic era that started when the first H1N1 emerged in 1918.
As soon as the newest H1N1 virus burst onto the scene in the spring, it conspicuously assaulted the young and left the old mostly unscathed. To date, 79 percent of confirmed U.S. cases have been in people younger than 30 years and only 2 percent in people older than 65. In light of that lopsided attack pattern, investigators at the Centers for Disease Control and Prevention quickly started testing hundreds of human serum samples stored between 1880 and 2000, looking for evidence of past human experience with the novel H1N1 virus.