Vaccination against polio began with the Salk vaccine in 1955. In 1952, nearly 3,000 people had died from polio and children in "iron lungs" populated wards in most hospitals. By 1991, however, the disease had been eradicated in the Western hemisphere. Iron lungs used to "breathe" for polio patients. They encase the thoracic cavity externally in an air-tight chamber. The chamber is used to create a negative pressure around the thoracic cavity, thereby, causing air to rush into the lungs to equalize intrapulmonary pressure. J. H. Emerson Co. manufactured this type of life saving respirator in the 1930s, but in the 1950's and '60's, the invention of portable ventilators made iron lungs obsolete. By 1970, J. H. Emerson Co. ceased manufacturing this apparatus, but many still depend on such mechanical ventilation systems on a daily basis. No date was given for the photograph.