The phenomenon, which was published in Nature this week, was discovered by Kimberly Seed and colleagues when they looked at bacteriophages who usually infect and kill the bacterium responsible for cholera Vibrio cholerae. When looking at samples, they noticed that the phages' genomes had elements of a particular set of genes that usually only certain kinds of bacteria have, called CRISPR-Cas, and that were recently found to be the basis of an adaptive immunity effective against phage infection. To test if the immune system helped the phage infect more V. cholerae, they first tried to infect the bacteria using phages that did not have the stolen CRISPR-Cas genes. Those phages were unsuccessful. But when the researchers added the immune-system-enhanced phages to a sample of cholera bacteria, the phages were successful in infecting and killing the bacteria. The phage does so by targeting bacterial genes that would normally inhibit the phage from infecting the cell. Without that defense, the bacteria quickly fills with copies of the virus particles and explodes.