ZombieMod: Source is an up-and-coming Orange Box modification for the Source engine. Currently, there are four different gamemodes, all with their own unique twist on the zombie apocalypse.

In Infection, all players are spawned as a survivor with a pistol and a broken bottle. After a random amount of time, one survivor is infected and becomes a zombie. It is then this player�s job to infect all of the other survivors with their zombie claws. It is only at this time that he or she will be able to declare victory. Conversely, it is the survivor�s job to fend off the zombie horde and stay alive until time runs out.

In Survival, all players are spawned as a survivor with a pistol and a broken bottle, just like in Infection. However, this is where the similarities end, as the survivors are then presented with waves of zombie NPCs, and are forced to defend themselves and stay alive until time runs out. Some Survival maps have allied NPCs on them as well, who aid the survivors in defeating the countless waves of zombie NPCs.

In Holdout, all players are spawned as a survivor with a pistol and a broken bottle, just like the other gamemodes. Similar to Survival, in Holdout the survivors are forced to defend themselves against hordes of zombie NPCs, however this time around there is an objective: prevent the zombie NPCs from getting to the holdout point, which is a location on the map marked by unique signs and textures. If the zombie NPCs manage to get to this location, the survivors will lose, so it is critical to the survivors� mission to hold out and camp the holdout point at all costs.

In Escape, all players are spawned as a survivor with a pistol and a broken bottle, upon which they are immediately forced to run like hell. In this gamemode, the survivors are forced to escape from a certain scenario, such as getting on a train to get away from a complex infected with zombies. Like Infection, a random zombie is chosen from the group of survivors after a random amount of time. The survivors� mission then becomes escape before time runs out or before the entire team becomes infected. Conversely, it is the zombies� mission to prevent the survivors from escaping.

ZombieMod: Source has several features that make it unique. There is a buymenu system, from which each survivor is allowed to purchase any weapon they desire, as long as they have the money to do so. Money is obtained by killing zombies, whether they are player zombies or zombie NPCs. There is also a complete pick-up and drop system for making barricades and for aiding allied players. Generally, when you spawn on a map, you have a choice: fulfill the objective with your teammates or abandon them in an attempt to go rogue. While these choices aren�t explicitly presented to a survivor, they are implied, and you will have to choose one before you can continue. These are the features that make ZM:S truly interesting and fun to play.