Salvador Edward Luria (1912 – 1991) was awarded one-third of the 1969 Nobel Prize in Physiology or Medicine, along with Max Delbrück and Alfred Day Hershey, for their discoveries concerning the genetic structure of viruses and how they replicate. In 1943, Luria published a paper with Delbrück showing that, contrary to what had been assumed, the genetic material of viruses undergoes permanent changes. That same year, he and Delbrück devised the “fluctuation test," which showed that genetic mutations are produced randomly in bacteria before they are selected. They provided experimental evidence that bacteria became resistant to phages (bacterial viruses) through spontaneous mutations rather than as a direct response to changes in the environment. In 1945, Hershey and Luria demonstrated the existence not only of such bacterial mutants but also of spontaneous phage mutants.