But one important theory is that what made it so deadly is that the flu virus in humans—to which we’d adapted some immunity—had combined with a genetic variant from birds. What this meant was that many human beings did not have an antibody to combat the flu, even among the massive pantheon of antibodies we all get born with. This is what the CDC says: “Influenza experts believe that a pandemic is most likely to be caused by an influenza subtype to which there is little, or no, preexisting immunity in the human population. There is evidence that some residual immunity to the 1918 virus, or a
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1918 Flu Pandemic
