In 1996 Matthew K. Waldor and John J. Mekalanos reported a stunning discovery about the toxin.
The toxin was for the first time shown to be not a part of the bacterium but actually that of a virus that got integrated into the V. cholerae genome.
Normally this virus remains silent within V. cholerae but during infection it gets activated.
The major virulence factor of cholera, CT (cholera toxin) is encoded on a filamentous phage (ctxΦ) that is capable of transducing the ctx gene into other cholera strains.
The released phages specifically attach to the bacterium and enter it. Vigorous viral multiplication results in the production of large amounts of toxin causing severe diarrhea.