The polio vaccine currently used in the United States, the Salk vaccine, consists of polioviruses that have been treated with chemicals so that they can no longer reproduce within host cells and thus cannot establish an infection. They are referred to as _______, rather than dead, viruses.

Polio is an infectious disease which can cause muscle paralysis or deformation of extremities. Caused by the poliovirus, it was considered a major health issue that had affected humanity for thousands of years, so efforts were made to develop a vaccine.

The first polio vaccines were developed around 1950, but they used live viruses. A major breakthrough occurred with the development of the inactive polio vaccine by Jonas Salk in 1955. Unlike other vaccines, which used weakened viruses (and thus still carried a risk of infection), the Salk vaccine used inactive viruses, which were rendered harmless through a chemical process that prevented them from reproducing within host cells. The Salk vaccine was an immediate success, with millions of children being immunized against polio in the United States and other industrialized states from the late 1950s onwards.

Polio vaccines based on the Salk vaccine have almost completely erradicated the disease, and today it's only present in some parts of Africa and the Indian subcontinent.