I was reading through next month's Discover, and there is an article covering the work of Rodolfo Acuna-Soto, who has proposed that some of the most major epidemics that killed the Aztecs and other ancient American populations may have been caused by an indigenous hemorrhagic fever. This 2000 paper and this 2002 paper lay out the case.
The abstract of Acuna-Soto et al. (2000) describes the disease:
In 1545, twenty-four years after the Spanish conquest of the Aztec empire, an epidemic of a malignant form of a hemorrhagic fever appeared in the highlands of Mexico. The illness was characterized by high fever, headache, and bleeding from the nose, ears, and mouth, accompanied by jaundice, severe abdominal and thoracic pain as well as acute neurological manifestations. The disease was highly lethal and lasted three to four days. It attacked primarily the native population, leaving the Spaniards almost unaffected. The hemorrhagic fevers remained in the area for three centuries and the etiologic agent is still unknown. In this report we describe, and now that more information is available, analyze four epidemics that occurred in Mexico during the colonial period with a focus on the epidemic of 1576 which killed 45% of the entire population of Mexico. It is important to retrieve such diseases and the epidemics they caused from their purely historical context and consider the reality that if they were to reemerge, they are potentially dangerous.
There are several lines of evidence leading to this interpretation, including contemporary accounts of symptoms by doctors and priests, the lack of correspondence of these symptoms to known epidemics like smallpox, typhus, yellow fever, malaria and the other usual candidates, the fact that the indigenous people used a distinct word, cocolitzi to describe the epidemics, which was different from their description of smallpox (often assumed to have been the cause), and the association between epidemics and droughts.
The papers suggest that the transmission pattern is similar to the hantavirus that caused Four Corners disease: years of drought followed by rainfall, which caused an explosion of rodent populations and greatly increased human-rodent contacts. They substantiate the hypothesis with analysis of tree-ring data, which show a correspondence between drought years and plague years, as well as a massive drought and wet sequence associated with the largest epidemic in 1545.
A big problem with the hypothesis could be the fact that the cocolitzi epidemics did not greatly affect Spaniards, despite the expectation that they should have had no immunity to a native American disease. Acuna-Soto and colleagues propose that this is a reflection of their high status -- they would have been under less stress during droughts than the indigenous people, and they might not have had as extensive contact with rodents.
The 2002 paper speculates that the epidemic may derive from either a hantavirus or an arenavirus, both of which have caused rodent-borne disease in the New World, but no known species of either has been found in the right place. Perhaps the closest analogue would be the Machupo virus that causes Bolivian hemorrhagic fever. It's an awful disease.
Will it return?
Cocoliztli was an emerging disease of its time and it appeared at a time of intense social and ecologic change. The illness ran without control and caused catastrophic damage to the Indian population for at least a century. Today, there are no reported human or animal diseases resembling cocoliztli in the area. The disease has not been reported for a long time and the probability of an epidemic reemergence
remains unknown. As for potential risk factors, it is important to remember that poverty, a key element in the epidemic, remains prevalent in some areas formerly affected by the disease. In the small towns around the city of Tehuacan, in the state of Puebla where cocoliztli once flourished, the word cocoliztli is still used as synonymous with lethal disease. Perhaps it is only representative of a historical vestige, but if the word and the concept remain active, we may well wonder if the etiologic agent is also alive and waiting to emerge again (Acuna-Soto et al. 2000:737).
It makes a good detective story, with quotes from unpublished historical documents, dendrochronology, and all the rest.
Acuna-Soto R, Romero LC, Maguire JH. 2000. Large epidemics of hemorrhagic fevers in Mexico 1545-1815. Am J Trop Med Hyg 62:733-739. PubMed
Acuna-Soto R, Stahle DW, Cleaveland MK, Therrell MD. 2002. Megadrought and megadeath in 16th century Mexico. Emerg Infect Dis 8:360-362. PubMed