Delbrück remained in the US during World War II, teaching physics at Vanderbilt University in Nashville while pursuing his genetic research. In 1942, he and Salvador Luria of Indiana University demonstrated that bacterial resistance to virus infection is caused by random mutation and not adaptive change. This research, known as the Luria-Delbrück experiment, was also significant for its use of mathematics to make quantitative predictions for the results to be expected from alternative models. For that work, they were awarded the Nobel Prize in Physiology or Medicine in 1969, sharing it with Alfred Hershey.