Salk, Jonas Edward (1914–1995), American immunologist. In 1942 while on a fellowship in epidemiology, Salk began studies of the influenza virus with the purpose of producing vaccines in commercial quantities. In 1947 he became the director of the virus research laboratory at the University of Pittsburgh, and two years later he changed his interest to developing a serum against poliomyelitis. A problem was presented by the fact that poliomyelitis is caused by three strains of virus and a single effective vaccine was needed that could neutralize all of the strains. In 1953 he announced the successful development of a vaccine prepared from virus inactivated by treatment with formaldehyde. In the next two years mass inoculation of school children with the Salk vaccine was undertaken. 1953 was also the year in which he published the results of experimental inoculations of 20,000 people with a flu vaccine which had produced immunity for as long as two years. That vaccine also went into wide use.