Hantaviruses are worldwide in distribution and cause two severe acute diseases of man, hemorrhagic fever with renal syndrome of the Old World and hantavirus cardiopulmonary syndrome of the New World. Chile is among the countries with the highest per capita burden of HCPS in the world in that in a population of about 14 million, it experiences typically 60 to 80 cases of HCPS due to Andes hantavirus per year, with a casefatality ratio of nearly 40%. Among the reasons that Chile experiences more cases than the US is that Andes virus, unlike the etiologic agents of HCPS in the US, is transmitted both by rodent reservoir hosts but also via interpersonal spread. As a result, in Chile there are manyfold greater numbers of "clustered" cases of HCPS in which a single case within a household is followed by secondary cases involving other persons, a phenomenon that is very rarely seen outside of South America. We have recently discovered that interpersonal spread of Andes virus among members of households greatly disproportionately affects the sexual partners of index cases while relatively sparing children and other household members. We believe that these cases can be prevented, but at present almost nothing is known about how they are occurring. We intend to conduct a prospective study of such secondary cases, those that involve sexual contacts of index cases. The goals are to determine the precise modes and risk factors for secondary spread of Andes virus among sexual partners of index cases, to understand what steps are involved in transmission and disease due to the virus, and ultimately to design new interventions to prevent further such tragic transmissions of this highly lethal infection.