Mix dead virulent bacteria with live harmless ones, and the harmless ones turn deadly

Frederick Griffith injected mice with harmless pneumococcus bacteria mixed with a virulent strain that had been killed with heat, expecting nothing to happen. The mice died anyway, and live virulent bacteria turned up in their blood afterward. Something from the dead cells had transformed the harmless strain into a killer, a 'transforming principle' that took another sixteen years to identify as DNA.

— Frederick Griffith, The Significance of Pneumococcal Types — Journal of Hygiene, 1928