Answer by taking a quote from the following article:

Jonas Edward Salk (; October 28, 1914 - June 23, 1995) was an American medical researcher and virologist. He discovered and developed one of the first successful polio vaccines. Born in New York City, he attended New York University School of Medicine, later choosing to do medical research instead of becoming a practicing physician. In 1939, after earning his medical degree, Salk began an internship as a physician scientist at Mount Sinai Hospital.

In 1947, Salk became ambitious for his own lab and was granted one at the University of Pittsburgh, but the lab was smaller than he had hoped and he found the rules imposed by the university restrictive. In 1948, Harry Weaver, the director of research at the National Foundation for Infantile Paralysis, contacted Salk. He asked Salk to find out if there were more types of polio than the three then known, offering additional space, equipment and researchers. For the first year he gathered supplies and researchers including Julius Youngner, Byron Bennett, L. James Lewis, and secretary Lorraine Friedman joined Salk's team, as well. As time went on, Salk began securing grants from the Mellon family and was able to build a working virology laboratory. He later joined the National Foundation for Infantile Paralysis's polio project established by President Franklin D. Roosevelt.  Extensive publicity and fear of polio led to much increased funding, $67 million by 1955, but research continued on dangerous live vaccines. Salk decided to use the safer 'killed' virus, instead of weakened forms of strains of polio viruses like the ones used contemporarily by Albert Sabin, who was developing an oral vaccine. After successful tests on laboratory animals, on July 2, 1952, assisted by the staff at the D.T. Watson Home for Crippled Children, Salk injected 43 children with his killed-virus vaccine. A few weeks later, Salk injected children at the Polk State School for the retarded and feeble-minded. In 1954 he tested the vaccine on about one million children, known as the polio pioneers. The vaccine was announced as safe on April 12, 1955.  The project became large, involving 100 million contributors to the March of Dimes, and 7 million volunteers. The foundation allowed itself to go into debt to finance the final research required to develop the Salk vaccine. Salk worked incessantly for two and a half years.  Salk's inactivated polio vaccine was the first vaccine for the disease; it came into use in 1955. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system.

When did he start doing Polio research?
In 1948, Harry Weaver, the director of research at the National Foundation for Infantile Paralysis, contacted Salk.