Hershey, Alfred Day (1908-1997) was an American biologist. He shared the 1969 Nobel Prize in physiology or medicine with the German-born American biologist Max Delbrück and the Italian-born American biologist Salvador Edward Luria for their work on bacteriophages, viruses that attack bacteria.

Hershey was born in Owosso, Michigan. He earned a bachelor's degree in 1930 and a doctorate in chemistry in 1934 from Michigan State College, now Michigan State University. From 1934 to 1950, he taught and did research in the bacteriology department at Washington University School of Medicine in St. Louis, Missouri.

Hershey, Delbrück, and Luria began exchanging information on research with bacteriophages, also called simply phages, in 1942. The three scientists became known as the “phage group.” In 1945, in separate experiments, Hershey and Luria discovered that both the phage and the bacterium undergo spontaneous mutation after the infection of the host cell. In 1946, Hershey discovered, and Delbrück independently confirmed, that genetic recombination took place in the viruses reproduced in the host cell.

In 1952, Hershey and his assistant, Martha Chase, conducted their famous blender experiment. Using an ordinary kitchen blender, they showed that only the nucleic acid (genetic material) of the bacteriophage was injected into the host cell upon infection and that the protein shell remained attached to the outside of the bacterium.

In 1950, Hershey joined the staff of the Carnegie Institution of Washington's Department of Genetics at Cold Spring Harbor, New York. The department was subsequently renamed the Genetics Research Unit, and Hershey served as its director from 1962 to 1971. Hershey died at his home in Syosset, New York, in 1997.