when was the flu discovered
History and Development of Flu Vaccines. In 1933, researchers discovered that viruses (influenza virus types A, B, and rarely C) cause influenza (flu). Prior to 1933, a bacterium named Haemophilus was thought to cause the flu.In 1938, Jonas Salk and Thomas Francis developed the first vaccine against flu viruses.This first flu vaccine was used to protect the U.S. military forces against the flu during World War II.rior to 1933, a bacterium named Haemophilus was thought to cause the flu. In 1938, Jonas Salk and Thomas Francis developed the first vaccine against flu viruses. This first flu vaccine was used to protect the U.S. military forces against the flu during World War II.