Shortly after, Hershey accepted a faculty position at Washington University in St. Louis,[1][2] serving as an instructor of bacteriology and immunology from 1934 to 1950.

At Washington University, Hershey worked closely with department head Jacques Bronfenbrenner to investigate bacteriophages, or phages—viruses that infect and replicate inside bacteria.

[3] Hershey's work on the factors impacting the virus' ability to infect its targets brought him to the attention of fellow phage researchers Max Delbrück and Salvador Luria.

Three years later, Hershey and Delbrück would independently discover that different strains of bacteriophage can both exchange genetic material when infecting the same bacterial cell.

Hershey's work with bacteriophage would earn him a share of the 1969 Nobel Prize in Physiology or Medicine with Delbrück and Luria, "for their discoveries concerning the replication mechanism and the genetic structure of viruses.