Frederick Griffith was a medical officer in Germany. 1928. In what is today known as Griffith's experiment, he discovered a transforming principle, which is today known as DNA.

Griffith was, in fact, trying to make a vaccine to prevent pneumonia infections in the epidemics after World War II. Griffith was trying to make a vaccine using two strains of the Stretococus pneumonia bacterium. The rough strain (R strain) did not cause pneumonia when injected into mice. The smooth strain (S strain) was deadly when injected, causing pneumonia and killing the mice in a day or two. When the S strain was heated to kill it and then injected into mice, it produced no ill effects in the subjects. However, when dead S coupled with live R were injected into the mouse, the mouse died. This is the first known example of bacterial transformation.