In 1928, scientist Frederick Griffith discovered the "transforming principle" while conducting experiments with Streptococcus pneumoniae bacteria. This principle showed that a nonlethal strain of the bacteria could be transformed into a disease-causing strain. Later, in 1944, researchers Oswald Avery, Maclyn McCarty, and Colin MacLeod identified DNA as the material responsible for this transformation. Their experiments showed that DNA, and not proteins, could transfer genetic information and change the properties of cells. This breakthrough surprised scientists and revealed the role of DNA as the hereditary material. Griffith was the first to demonstrate this phenomenon, which showed how healthy cells could turn into harmful ones.