This document summarizes information about Sin Nombre virus (SNV) which causes Hantavirus Pulmonary Syndrome (HPS). SNV is transmitted to humans from its primary reservoir, the deer mouse, through inhalation of excreta or saliva particles. It has a relatively low mutation rate for a RNA virus. HPS has two phases, an initial flu-like phase followed by rapid onset of breathing problems and fluid in the lungs. The mortality rate for HPS is 35-40%. The first recognized outbreak in the US occurred in 1993 associated with heavy snowfall. There are currently no vaccines or specific treatments, though cardiopulmonary support can improve survival rates.