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 asssumed to be 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 brains, either using melee attacks or firearms. Cutting or shooting off their limbs does nothing but reduce their mobility and danger. Exception: the low slice attack slices off both legs off non-freak zombies. They become totally harmless as a result (in the first game), and bleed out and die in about 3 seconds.
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 found 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 SitReps 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. Furthermore, the Capital is the destination of all the High Value Targets and refugees which are extracted from Danforth. The SitReps also tell of uncomfirmed reports that Air Force One has crashed and that the president of the United States has been killed.
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. It is headed by 1 to 3 Screamers and 5 or more zombies. Their numbers could grow once a Screamer shrieks to summon more zombies from the vicinity.
Passing Zombie Horde(s) can even join the crowd but uncounted as members of an Infestation. After losing interest either from the call of a Screamer or undetected survivor, these hordes would bolt out of the structure and resume their halted parade. They are however can be dangerous for players choosing a frontal attack as their numbers are greater even with a follower on their side.
The best way to deal an infestation is by luring the members out of the building with some firecrackers or even explosives. If crossbows are unavailable to the inventory, prefer to use molotov or flame grenade to initially kill the crowd instantly upon luring; and if there are still remaining zeds inside, make use of stealth and strike them quickly. After killing the last zombie, the infestated structure symbol will be removed from the mini map.
An infestation commonly hole themselves inside a structure, yet sometimes it's not. On State of Decay 2's initial game and even after many updates, most or sometimes the entire members are outside the building but stays inside its perimeters. There are also instances of infestation that gets cleared without doing anything, even just by passing the area.
Even on some rarest moment that the last remaining zombie is hidden from sight, by returning to main menu and continuing the game will the zombie reappear inside the infested structure.
"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.
The introduction of Plague types on State of Decay 2 have different type of hordes into normal zombie and plague zombie horde; a mixed horde isn't possible. They appear in random areas parading from place to place. Their numbers will increase drastically upon the game length, but some considers their increasing numbers from growing infestation.
Killing a member of the horde alerts the others, but they will lose interest if they cannot detect the survivor. Nevertheless, a lone zombie joining a horde is not possible.
Unless the player chose to opt into stealth, frontal attacks are more dangerous. The survivor must be armed with fully repaired range weapon (or quieter guns especially .22 firearms) , and mastered a fighting style. This approach have numerous setbacks such as:
- The possibility to attract more zombies in vicinity even other horde(s) or freak(s) which are far more lethal type of zed.
- Guns, especially non-suppressed guns even .22 firearms, have corresponding noise radius noticed on the mini-map. Either using it by mid range or for replenishing stamina, has the same setback with the first condition.
- Using any explosives while a survivor is trapped injures them, it decreases their potential for melee attacks and lower hp bar.
Exterminating a horde in stealth are better even if the sole priority is them alone. Suppressed guns like sighted ones are the ideal weapon of choice but having an Advanced Suppressor is the best. Sighted crossbows are also preferred yet the player needs better accuracy and timing. Unless the player wants to be undetected with crossbows, the survivor is better when in prone. This approach have the advantages of:
- Undetected kills with crossbows.
- Undetected kills with suppressed firearms, but these weapons still have noise radius yet minimal and can still be heard by zeds at the radius.
- Freaks can be dispatched easily.
- Throwing molotovs to the horde kills them instantly.
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.