Luria, Salvador Edward (1912-1991), an Italian-born American biologist, shared the 1969 Nobel Prize for physiology or medicine with German-born American biophysicist Max Delbruck and American biologist Alfred Hershey for their work with bacteriophages (viruses that attack bacteria). In 1943, Luria and Delbruck showed that the genetic material of viruses undergoes changes in successive generations similar to those that occur in more complicated organisms. They had previously thought that viruses had only one gene which was replicated.