In the aftermath of the deadly 1918 flu epidemic, governments across the globe rushed to develop vaccines that could stop the spread of infectious diseases. In England, microbiologist Frederick Griffith was studying two strains of Streptococcus pneumoniae that varied dramatically in both their appearance and their virulence, or their ability to cause disease. Specifically, the highly virulent S strain had a smooth capsule, or outer coat composed of polysaccharides, while the nonvirulent R strain had a rough appearance and lacked a capsule (Figure 1).

Mice injected with the S strain died within a few days after injection, while mice injected with the R strain did not die.