It is generally accepted history that the Spaniards and other Europeans brought diseases with them that killed off the indigenous people of the Americas. There is certainly some truth to that in some areas of the New World -- but it may not be the case in Mexico, which is usually cited as Exhibit A for the "foreign diseases" argument.

Here's what history tells us about the Spanish conquest of Mexico: 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 may have killed the bulk of Mexico's native population, which plummeted from an estimated 22 million in 1519, when the Spaniards arrived, to 2 million in 1600.

And he warns that the fever — which the Aztecs called cocoliztli in their Nahuatl language — may still be lurking in remote rural areas of Mexico.

Not everyone buys the theory. But Acuña-Soto, who spent 12 years poring over colonial archives, census data, graveyard records and autopsy reports, is convinced that many historians are wrong about what killed the Aztecs.

"The problem with history is that it's very ideological," he said. "In this case, it was a beautiful way of accusing the Spaniards of unimaginable cruelties and of decimating the population of Mexico."

Spanish colonizers were far from blameless, he quickly points out. By subjecting the Indians to slave-like conditions and malnutrition, they made them more vulnerable to the disease, he said.

"Of course, there's a terrible story of cruelty and disease that killed a huge amount of indigenous people," he said. "But we don't know what this disease was."

Acuña-Soto, who has published his findings in several international scholarly journals, is a research professor at Mexico's National Autonomous University.

Notice, this doesn't absolve teh Spanish of charges of great cruelty. But it does raise the possibility that something else was at work -- and that the agent of infection is still around. I ca't wait to se what further research shows.