When the Digital World became so overpopulated, the super computer that governs the Digital World, Yggdrasill, wasn't able to handle it anymore. His solution was to wipe out the vast majority of the Digimon with the X-Program, and choose a very small percentage to be moved to a new Digital World, and destroy the old one. Those who were not chosen but still survived and moved to the new world had a rare gift known as the X-Antibody. This antibody, when activated, changed their appearance and strength.