Explanation:

Salk vaccine: Vaccine against poliomyelitis named for Dr. Jonas Salk who developed and introduced it in 1955. It was the first type of polio vaccine to become available. It was made by cultivating three strains of the virus separately in monkey tissue. The virus was separated from the tissue, stored for a week, and killed with formaldehyde. This killed-virus vaccine was given by injection and required 4 "shots." The oral form of the vaccine, subsequently developed by Dr. Albert Sabin, is in standard use today since it is easier to administer and is more effective than the Salk vaccine. The Salk vaccine is now exclusively of historic interest.