logical testing and autopsy findings did not indicate any known cause of acute respiratory illness and shock. Persons who were affected lived in rural areas of northwestern New Mexico and northeastern Arizona, but there was no obvious epidemiologic connection between patients. Recognition and reporting of this outbreak led to an unprecedented multiagency response that involved the collaborative efforts of clinicians, field epidemiologists, biologists, laboratory researchers, and public health educators. Specimens from the patients were tested for a wide variety of agents at the Centers for Disease Control and Prevention (CDC) [1]. Within 3 weeks of the recognition of the outbreak, the first evidence of the cause of illness was found. IgM and IgG antibodies to several hantaviruses were detected in the sera of patients with use of enzyme immunoassay (EIA), suggesting that illness was due to a previously unrecognized but serologically cross-reactive hantavirus. Additional evidence that a hantavirus was the etiologic agent accumulated in short order. Antigen was detected in the endothelia of multiple organs of these patients on immunohistochemical testing when a monoclonal antibody reactive with conserved hantaviral nucleoprotein epitopes was used, and hantaviral nucleotide sequences were detected in patient tissue specimens by reverse transcriptase polymerase chain reaction (RT-PCR). Nichol and colleagues [2] confirmed that the etiologic agent was a unique hantavirus on the basis of results of nucleotide sequence analysis of RT-PCR-amplified viral genetic material. The respiratory illness associated with hantavirus infection has been designated hantavirus pulmonary syndrome (HPS) [3]. Clinical illness associated with hantavirus infection first