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Frederick Griffith (1879 - 1941) was a British medical officer. In 1928, in what is today known as Griffith's experiment, he discovered a transforming principle, which is today known as DNA. Griffith was trying to make a vaccine to prevent pneumonia infections in the epidemics after World War I. Griffith was trying to make a vaccine using two strains of the Streptococcus pneumoniae bacterium. The rough strain (R strain) did not cause pneumonia when injected into mice and was not covered with a polysaccharide capsule. The smooth strain (S strain) was deadly when injected, causing pneumonia and killing the mice in a day or two and does have a polysacchride capsule. When the S strain was heated to inactivate 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. After isolating bacteria from the blood of the mice, Griffith discovered that the normally nonpathogenic R bacteria had acquired polysaccharide capsules. The bacteria isolated from the mice infected with the mixture of live R and heat inactived S were all of the S strain, and maintained this phenotype over many generations. Griffith hypothesized that some "transforming principle" from the heat inactivated S strain converted the R strain to the virulent S strain. It wasn't until several years later that Griffith's "transforming principle" was identified as DNA. Griffith was killed at work along with longtime friend and bateriologist William M. Scott in London as a result of an air raid. He was the uncle of John Stanley Griffith, a winner of the Royal Society's Faraday Medal.
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