Zombie Modifier

Zombie Modifiers are buffs that can be randomly applied to zombies in. They will often drastically alter approaches to killing a zombie, either by forcing a weapon switch due to resistances or by causing absolute havoc within its reach with upgraded damage capabilities.

Modified Zombies can be identified by the visual effects around the zombie if the VFX option is turned on, or by hovering the cursor over the modified zombie, revealing the word "Elite" after the zombie name, exactly which buffs the zombie has, and distinctive orange text.

Only Zombie Elites can have buffs, and Zombie Elites occasionally spawn within unmodified zombies. The amount of buffs a zombie has increases as the zombie evolves. The "Fetid" variant can have exactly one buff, the "Evolved" variant exactly two, and the "Apex" variant three.

Elite Zombies also have about 50% more health than normal.

The full list of modifiers is as follows:

Fiery Skin

 * 75% thermal resistance, +100% fire damage.

Poisoned Limbs

 * 75% poison/acid resistance, 100% acid damage over 3 sec.

Chitinous Hide

 * 75% physical resistance.

Extra Fast

 * +50% movement speed. Formerly was white silhouettes around the zombie, but now wind trails appear around the zombie. The Enemy Stats section still displays the old image.

Berserk

 * Tripled attack speed. Formerly was red silhouettes around the zombie, but now a red ring appears around the zombie. The Enemy Stats section still displays the old image.

Extra Strong

 * Double damage on all attacks, including melee attacks, Spitter acid, damage over time from Fiery Skin or Poisoned Limbs, and Acid Blood. A red aura appears around the zombie.

Regenerating

 * The zombie regenerates 5% of its health per second.

Energy Resilient

 * 75% energy resistance.

Dark Minion

 * The infected functions similar to the Shadow of SAS 3. They ignore all obstacles and ignore damage reduction from armour by 75%, though Heavies with the Tough Body skill can still reduce their damage. The zombie is still visible on the radar.

Acid Blood

 * On death, the zombie creates a puddle of acid that lasts 10 seconds. On contact with the acid, 150% damage over 3 sec. The acid lasts 3 sec. after getting out of the puddle. Acidic bubbles appear around the zombie.

Nightmarish

 * 10x health and damage, 1.5x size, which adds 50% to attack range. Includes all modifiers except Dark Minion, Acid Blood, and Regenerating. After the June 30, 2015 update, nightmarish zombies now ignore 5% of armor damage reduction.
 * On mobile, this property can also appear on elite bosses in nightmare mode, making them vastly stronger.
 * Nightmarish bosses: All bosses in Nightmare mode deal more damage, have four times as much health, and are larger in size, which adds to their attack range, even being able to hit players beyond some thinner walls. If they do, bosses come with only the normal buffs for bosses (extra fast, dark minion, berserk, or extra strong).

Modifier Combinations
After the June 19, 2014 update, each zombie type in a mission has two types of "Elite" variants, with set combinations of up to three separate modifiers. Bosses can receive modifiers as well, but they are limited to only one of: Berserk, Extra Strong, Extra Fast, or Dark Minion. Before the update, it was just random modifiers on any zombie, and the bosses did not have any modifiers.

Some specific combinations can be absolutely nightmarish to face, a few are as follows:
 * Bloaters can receive the same modifiers any regular infected can. This means that:
 * An Extra Strong Bloater Elite has massively increased damage, given that the Bloater Elite's increased damage is doubled.
 * An Extra Fast Bloater Elite can corner you quickly if you're not careful.
 * A Bloater Elite with Berserk not only makes it deadly to encounter when cornered, but can destroy barricades much quicker than any other zombies.
 * A Regenerating Bloater Elite makes it much harder to kill, due to the increased Elite's health and the Bloater's already high health.
 * Bloater Elites with resistances (e.g.: Chitinous Hide) become almost impossible to kill with the resisted damage type, whereas weaker zombies with resistances would take a few more shots than usual to kill.
 * Fiery Skin or Poisoned Limbs Bloater Elites not only give them a resistance, but makes their attacks hit harder and deals damage over time. Since damage is based on a percentage of the zombie's damage, a hit from a Bloater Elite with these buffs will hurt much more than any other zombie with the same buffs.
 * Acid Blood from a Bloater Elite becomes extremely deadly as again, acid blood damage is based on zombie damage.
 * A combination of any of the above (ex: Extra Strong + Acid Blood, Chitinous Hide + Regenerating, Dark Minion + Extra Strong + Extra Fast).


 * Zombies with resistances and the Regenerating modifier can be extremely difficult, if not impossible to kill with the resisted damage type(s).
 * Apex Spitters Elite with Berserk, Poisoned Limbs, and Acid Blood, can make players who lack chemical resistance want to rip their hair out.
 * A group of Extra Fast Runners can easily overrun players, especially when combined with resistances and Dark Minion.
 * Almost any zombie will turn into a killing machine when applied with both Extra Strong and Berserk, providing them with six times the effective damage of a regular zombie. Extra Fast on top of that will turn them from dangerous to lethal, or the Dark Minion Buff, making players take the full amount of damage the zombie would inflict.
 * Extra Fast bosses can be lethal even if it's in multiplayer. Because it's the best tactic to kite (drag), shoot, and repeat on bosses, extra fast bosses can quickly catch up to you while shooting at it if the player doesn't have concussion rounds or cryo turrets.
 * Zombies with any buffs other than Chitinous Hide will have doubled health.
 * Shielders with Chitinous Hide will be a pain to kill with a physical weapon and are near-impossible to kill with physical weapons without Adaptive only if they are an Apex Regenerator.
 * Dark Minion zombies can be painful, as they ignore barricades, other obstacles, and any resistances, making it harder to kill, especially if combined with other modifiers.