MEXICO CITY — Here's what history tells us about the Spanish conquest of this country: Armed with modern weapons and old world diseases, several hundred Spanish soldiers toppled the Aztec empire in 1521. And by the end of the century, the invaders' guns, steel and germs had wiped out 90 percent of the natives. It's a key piece of the "Black Legend," the tales of atrocities committed by the Spanish Inquisition and colonizers of the New World. But it may be just that — legend, according to Rodolfo Acuña-Soto, a Harvard-trained epidemiologist. He argues that an unknown indigenous hemorrhagic fever...