A phrase denoting the occurrence of hantavirus pulmonary syndrome (HPS), a rare but potentially fatal respiratory disease caused by hantaviruses (primarily Sin Nombre virus) transmitted through inhalation of aerosolized urine, droppings, or saliva from infected rodents like deer mice, with around 890 cases reported in the United States since surveillance began in 1993.

Primarily affects rural and outdoor areas in the western U.S., where rodent populations are high; symptoms include fever, muscle aches, and rapid progression to respiratory failure, with a case fatality rate of about 36%.