The reanimated pathogenic-vectors (i.e. zombies) are the non-human antagonists in State of Decay. It isn't known what exactly created them, but it is assumably pandemic-related (due to the description of the Hazmat hero). Their behavior widely varies (there are walkers, runners, etc.). Some become mutated, taking distinctive appearances and characteristics; these special zombies are known as "Freaks".
The only way to kill zombies when on foot is to destroy their brain, either using melee attacks or firearms. Cutting or shooting off their limbs does nothing to reduce their mobility and danger.
A sufficiently powerful attack might knock a zombie to its knees, or on its back, making it briefly vulnerable to executions. However, if the survivor misses this window, the zombie will regain its balance and continue attacking.
The Situation Reports which can be read in Lifeline give various insights about the zombie pandemic. It is revealed that the undead have besieged the whole of North America in a matter of weeks and other countries around the globe are also suffering the same fate, with Russia and Ukraine going as far as launching their nuclear warheads at each other. The Reports also tell of conditions in various cities, such as Chicago which is assumed to have been overrun and destroyed, and Washington D.C, which is possibly one of the last population centers still standing in the country.
Eighteen months after the outbreak, the zombie virus seems to have mutated further. Survivors are shown dying and reanimating without ever having been bitten, suggesting that the pathogen may be airborne and may only trigger upon death. In addition, a new, much deadlier disease known as blood plague has appeared.
Occasionally, a large number of zombies will gather in a random location, creating an Infestation.
"Hordes" are groups consisting of 5-20 zombies that travel together as a hive-minded undead unit. These are extremely dangerous for both the player and the other survivors in Trumbull Valley.
In State of Decay a number of special zombie sub-types exist. Often called "Freaks", these zombies possess unique properties that regular zombies don't have.
Due to the stealth system in place in SoD, zombies rely on an "aggro system" to detect a survivor and/or playable character. Several functions affect detection. Such as line of sight, sound, lighting, movement and whether or not there is the presence of a player used distraction. Smell will not be taken into account in detection.
In larger affect is a Horde system, over time, base and outpost activity will cause zombies to spawn in the region around the base, if the problem is not dealt with, such as clearing out infestations, a horde will attack the base and attempt to destroy it.
To reduce lag and processes in the game, depending on the distance different animations and AI will take place, zombies beyond/out of player sight will have very simple AI with movement and detection in place. As the player comes into sight the AI will become more advanced, with a more advanced detection and movement system. Once in combat range of a player, the AI will be in its most advanced form, with all mechanics and animations in place.
- In State of Decay 2, "joke" zombies will occasionally spawn in addition to regular undead. These "joke" zombies include undead clowns and zombies wearing traffic cones as hats.
- Undead Labs has confirmed that the "black fever" storyline will no longer be explored, instead focusing on the blood plague and its effects on the zombie apocalypse. How this will affect future installments is as of yet unknown.