Oswald Avery's main contribution to science is the discovery that DNA is the active agent in genetic transformation, which he called the "transforming principle," according to Cracking the Code. This principle started the modern phase of DNA research.
Avery and his fellow scientists at the Rockefeller Institute in New York made the discovery while searching for a cure for pneumonia. Using recent information that one type of pneumonia could transform into another in vivo, Avery aimed to identify the conditions wherein transformation could occur and the agents that could make these changes possible. Avery and his research group then initiated the process of identifying DNA as the agent of change among living things. Eventually, they discovered the "transforming principle," according to Cracking the Code, and the discovery was announced in the New England Journal of Medicine in 1944.
Matthew Cobb, a zoology professor at the University of Manchester, discusses in the Guardian how Avery discovered a mysterious substance in the pneumonia bacteria, an organism which he had been studying for 30 years. Based on information provided by British medical officer Fred Griffith about the transformation of noninfectious forms of pneumonia into dangerous, virulent forms, Avery successfully discovered that DNA is an active agent in transformation, which made an impact on the field of molecular biology.