In May 1993, an outbreak of an unexplained pulmonary illness occurred in the southwestern United States, in an area shared by Arizona, New Mexico, Colorado and Utah known as "The Four Corners." A number of previously healthy young adults suddenly developed acute respiratory symptoms; about half died. Researchers soon suspected that they were dealing with a form of hantavirus, which is transmitted by rodents. Investigating the possible rodent connection, researchers trapped rodents in the affected area until the virus and its principal carrier — the deer mouse — were positively identified. By November 1993, the specific virus that caused the Four Corners outbreak was isolated. The new virus was called Sin Nombre virus and the new disease caused by the virus was named hantavirus pulmonary syndrome, or HPS.
Hantavirus is present in the saliva, urine, and feces of infected mice. People are infected by breathing in the virus during direct contact with rodents or from disturbing dust and feces from mice nests or surfaces contaminated with mice droppings or urine.