A microbiologist named Frederick Griffith studied Streptococcus pneumoniae, a bacteria that causes the the deadly lung disease, Pneumonia.
Griffith compared two forms of the disease, the S-strain which was highly pathogenic but could be made non-pathogenic by heating it, and the R-strain which was non-pathogenic.
He created an experiment in which he injected different strands, and mixtures of strands of the bacteria into mice and observed what happened to them.
Griffith noticed that the mice died after being injected with the living S-strain bacteria, as well as a mixture of heat-killed S-strain bacteria and living R-strain bacteria, while the mice lived when the R-strain bacteria and just the heat-killed S-strain bacteria were injected.
He concluded that the S-strain had somehow passed on its deadly properties to the live, non-pathogenic R-strain. He called this phenomenon, "The Transforming Principle".