Hemorrhagic fever with renal syndrome (HFRS) is a rodent-borne zoonosis endemic across much of Europe and Asia that is responsible for considerable human morbidity and mortality. A spectrum of clinical severity is recognized with HFRS, ranging from asymptomatic or mild infection to fulminant hemorrhagic fever and death. The etiologic agents are a group of at least 5 antigenically distinguishable viruses comprising the Hantavirus genus of the large virus family Bunyaviridae. These viruses establish chronic infections in certain species of rodents and are transmitted to man primarily via aerosols or fomites from contaminated excreta. Geographic (and, to a large degree, clinical) variation is determined by the distribution of the rodent species associated with each distinct virus.