Two major groups of hantaviruses are recognized based on clinical presentation. The first group includes Sin Nombre Virus (SNV), which causes hantavirus pulmonary syndrome, a severe and sometimes fatal form of acute respiratory distress. A second group of hantaviruses (including Seoul, Hantaan, Dobrava, and Puumala) causes hemorrhagic fever with renal syndrome, a condition not typically seen in the United States. Sera are initially screened for IgG and IgM antibodies recognizing the nucleocapsid protein common to all hantaviruses. All Hanta IgM positive samples from US residents will be sent to a Public Health Laboratory for SNV-specific IgM. Samples that are Hanta IgG positive but IgM negative are not subjected to SNV-specific IgM testing, since the lack of IgM rules out acute SNV infection. A positive Hanta IgM result but a negative SNV-specific IgM antibody result may indicate either reactivity to a hantavirus other than SNV or false positive reactivity. A small number of SNV IgM postive (but Hanta IgG negative) samples represent false positive reactivity associated with acute cytomegalovirus or Epstein Barr virus infection.