In 1918, an influenza virus jumped from birds to humans and killed an estimated 50 million to 100 million people in a world with less than a quarter of today’s population. For years it has been devastating bird populations worldwide and more recently has begun infecting mammals, including cattle, a transmission never seen before. In another first, the virus almost certainly jumped recently from a cow to at least one human — fortunately, a mild case. As they do, they must be cautious about the lessons they might think Covid-19 left behind. Two assumptions based on our Covid experience would be especially dangerous and could cause tremendous damage, even if policymakers realized their mistake and adjusted quickly.

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