Influenza
History

From 1918 to 1920, the Spanish flu pandemic became the most devastating influenza pandemic and one of the deadliest pandemics in history. The pandemic, caused by an H1N1 strain of influenza A, likely began in the United States before spreading worldwide via soldiers during and after the First World War. The initial wave in the first half of 1918 was relatively minor and resembled past flu pandemics, but the second wave later that year had a much higher mortality rate. A third wave with lower mortality occurred in many places a few months after the second. By the end of 1920, it is estimated that about a third to half of all people in the world had been infected, with tens of millions of deaths, disproportionately young adults. During the 1918 pandemic, the respiratory route of transmission was clearly identified and influenza was shown to be caused by a "filter passer", not a bacterium, but there remained a lack of agreement about influenza's cause for another decade and research on influenza declined. After the pandemic, H1N1 circulated in humans in seasonal form until the next pandemic.
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