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In 1993 Sin Nombre virus was recognized as the cause of hantavirus pulmonary syndrome (HPS) and the deer mouse (Peromyscus maniculatus) was identified as the reservoir host. Surveillance by the Centers for Disease Control and Prevention and state health departments includes investigation to determine the likely site(s) and activities that led to infection, an environmental assessment of the home and workplace, and possibly rodent trappings at these sites. As of December 31, 1998, there were 200 confirmed cases from 30 states (43% case-fatality ratio). The national HPS case registry was examined to determine the incubation period of HPS. Review of 11 case-patients with welldefined and isolated exposure to rodents suggests that the incubation period of HPS is 9 to 33 days, with a median of 14-17 days. Case investigations allow a better understanding of the incubation time of HPS and may define highrisk behaviors that can be targeted for intervention.

Hantavirus pulmonary syndrome (HPS) occurs in most infections with Sin Nombre virus and other North American hantaviruses. We report five cases of acute hantavirus infection that did not fit the HPS case definition. The patients had characteristic prodromal symptoms without severe pulmonary involvement. These cases suggest that surveillance for HPS may need to be expanded.

The initial identification in 1993 of hantavirus pulmonary syndrome as a novel, highly fatal respiratory illness among American Indians in the southwestern USA in 1993 opened the window to the recognition of a well-established pan-American zoonosis with a myriad of causative viruses and rodent vectors, although all are New World hantaviruses among New World sigmodontine rodents. The clinical spectrum of symptoms has also been expanded to include asymptomatic infection through to fulminant hemorrhagic fever. Although the use of ribavirin, an antiviral drug, was disappointing in an early, open-labeled trial, early detection and supportive care is much better refined. However, much work remains in probing the pathogenesis of this syndrome to help define and explore therapeutic options and the mechanism of person-to-person transmission with Andes virus, one of the viruses that cause hantavirus pulmonary syndrome. Current remote sensing efforts and longitudinal ecologic investigations need to be expanded in order to focus prevention efforts better.

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