Mexicans have long been taught to blame diseases brought by the Spaniards for wiping out most of their Indian ancestors. But recent research suggests things may not be that simple. While the initial big die-offs are still blamed on the Conquistadors who started arriving in 1519, even more virulent epidemics in 1545 and 1576 may have been caused by a native blood-hemorrhaging fever spread by rats, Mexican researchers say. The idea has sparked heated debate in Mexican academic circles. One camp holds that the epidemics could have been spread by rats migrating during a drought cycle; others say newly arrived Spanish miners may have disturbed the habitat of virus-carrying rodents while searching for gold and silver. The revisionists draw support from one of the only authoritative firsthand accounts of the epidemics, a text lost for hundreds of years until it was found, misfiled, in a Spanish archive. Dr. Francisco Hernandez, a physician to the Spanish king who witnessed the epidemic of 1576 and conducted autopsies, describes a fever that caused heavy bleeding, similar to the hemorrhagic Ebola virus. It raced through the Indian population, killing four out of five people infected, often within a day or two. "Blood flowed from the ears and in many cases blood truly gushed from the nose," he wrote. "Of those with recurring disease, almost none was saved." Harvard-trained epidemiologist Dr. Rodolfo Acuna-Soto, a microbiology professor at Mexico's National Autonomous University, had Hernandez' work translated from the original Latin in 2000. He followed up with research into outbreaks in Mexico's isolated central highlands, where indigenous rats may have spread the disease through urine and droppings. Acuna-Soto's theory - which has been published in several scientific journals, including Emerging Infectious Diseases and theAmerican Journal of Tropical Medicine and Hygiene - runs counter to the belief that most of Mexico's Indian population died of Spanish-imported diseases such as smallpox, to which their bodies had no immunity. "This wasn't smallpox," Acuna-Soto says. "The pathology just does not fit." He says some historians in Mexico are offended by his theory. "Much of the reason why these epidemics were left unstudied was that it was politically and institutionally easier to blame the Spaniards for all of the horrible things that might have happened," he said. "It was the official version of history." Certainly, imported diseases such as smallpox, measles and typhoid fever did cause huge numbers of deaths starting in 1521. But the epidemics of 1545 and 1576 struck survivors of the first die-offs and their children, who would presumably have developed some immunity. While there is no reliable figure on Mexico's population in the 1500s - estimates range from 6 million to 25 million - it is clear that by 1600 only around 2 million remained. The epidemic "was so big that it ruined and destroyed almost the entire land," wrote Fray Juan de Torquemada, a Franciscan historian who witnessed the epidemic of 1576, adding Mexico "was left almost empty." "Many were dead and others almost dead, and nobody had the health or strength to help the diseased or bury the dead." Other accounts speak of a rodent invasion, and Acuna-Soto teamed up with US researchers to investigate whether an abnormally severe drought may have pushed rats into human settlements or vice versa. But another Mexican expert in the field insists the rodents mentioned in texts from the era probably came from Europe or Asia carrying the bubonic plague, which sometimes caused its victims to vomit blood. Elsa Malvido, a demographer, historian and an expert on ancient epidemics for the National Institute of Anthropology and History, says the plague could have caused the more severe hemorrhagic symptoms recorded by Hernandez, because it was attacking a population with no immunity whatsoever. But Dr. Carlos Viesca, director of medical history at Mexico's national university, says he is close to being convinced the epidemics were native. "The problem didn't start in Acapulco or Veracruz," the two main seaports where rats would have landed from overseas, he said. Instead, the disease appears to have started in the central highlands at a time when the Spaniards sent mining expeditions to unsettled parts of Mexico, raising the possibility that humans invaded rodent habitats, he said. Relatively few Spaniards were affected by the outbreak, possibly because in either eventuality they were protected: If the cause was bubonic plague or smallpox, their bodies had greater immunity to it; and if it was rodent-borne, they were less likely to come into contact with the animals.