full article, see zombie infestation
In summary, planet Zelk contained the first zombies which came from the mysterious atmosphere it possess. For thousands of years, the zombies had been under control and weak. This all changed when the zombies found a nuclear testing ground. Because of the radio-active waves, it increased the zombies power and they then started a massive war on the planet. After fighting for some time, a group of zombies stole away on a vertical-launch escape pod. A little later they arrived on Star Cruiser no. 13.