Four times in the past century, a new strain of flu has emerged that can spread quickly in humans. One of those strains, which emerged in 1918, killed an estimated 50 million people.
All human flu strains evolved from flu viruses that live in birds. To understand how these transitions happen, scientists have recently been tinkering with a strain of bird flu to see how many mutations it takes until its spreads from mammal to mammal.
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