Past vaccinations and previous infection by interrelated viruses may account for the mildness of the new H1N1 swine flu. Bettmann CORBIS
From Scientific American:
How the legacy of the vicious 1918 outbreak led to today's comparatively tame swine flu.
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.
Read more ....