Cholera bacteria, Vibrio cholerae, exists in a harmless form and a disease-causing virulent form. In the latter form, it causes the deadly disease cholera, but how the bacteria changed from harmless to deadly was not known until recently. Research now shows that a bacteriophage that infects V. cholerae transfers a gene that codes for the cholera toxin. This gene is then incorporated into the bacterial chromosome, thereby transforming the benign bacteria to a disease-causing agent. The transfer occurs through bacterial pili, and in further experiments, mutant bacteria that did not have pili were resistant to infection by the bacteriophage. This discovery has important implications in the development of vaccines against cholera, which have been unsuccessful up to this point.
Source: "Phage Transfer: A New Player Turns Up in Cholera Infection" by Nigel Williams, Science, June 28, 1996