Zombies is a term used for a X.A.N.A. possessed human. It is generally used in place of "X.A.N.A.-Name" when it is a group of people or someone who is not commonly possessed (like background characters).
Zombies were first seen in Ghost Channel where X.A.N.A imprisoned them and could make zombies at will. They were next seen in Attack of the Zombies where Kiwi was possessed and bites people to turn them into zombies.
Zombies have their own way of turning people into them by biting; although this is never seen but is proven. The only people that were not zombie-fied are Ulrich, Yumi, Sissi, Milly, Aelita and Rosa, the Kadic chef. Zombies have altered voices that sound malevolent in nature.