Since the original publication of this book, scientists have found evidence (the question is not settled) that seven of the eight segments of the 1918 virus are of avian origin, and the virus jumped species to humans probably after a reassortment (see 112) with another virus in which it acquired a human hemagglutinin gene—the gene which allows the virus to bind to and thus infect cells. And even that eighth segment had recent avian roots. This reassortment would have occurred when the avian virus infected a mammal—human, horse, pig, whatever—that was simultaneously infected by another
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