Rayman1103's collection of mutation mods is amazing, but it offers no explanation as to what each mutation actually does. Although there exist a few annotations on the internet, the ones I found were not user-friendly at all, so I took the liberty to create this list. The mutations are categorized in the same way as they are in the game. If you see any mistakes, feel free to correct them in the comments below.

Note: Throughout the guide, you may see the term Leaker pop up. If you are unaware, Leakers were a type of Special Infected that was eventually cut from the final game, and were a cross between Spitters and Boomers. They would rush the player and explode, causing the ground to catch fire. Both the explosion and fire would deal damage to the player.

Low Gravity. M60 only. Infinite ammo.

Boomers only. Some Boomers behave like Leakers. M60 only. Infinite ammo.

No Common Infected. Double Special Infected.

Low Gravity. Molotov only. Infinite ammo. No fire damage. No incapacitations.

Hunters only. High spawn rate. Hunters don't pounce.

If no one is holding the Gnome for cumulative 45 seconds, everyone dies.

Common Infected are slow. Headshots only. No incapacitaions. When health reaches 0, you have 2 minutes to reach the SafeRoom or die.

Gib Fest. No Common Infected. Special Infected die easily. High spawn rate. No incapacitations.

Common Infected only. When Common Infected die, there is a chance to spawn a Special Infected in its place.

No Common Infected. Dying Special Infected split into two more.

Boomers only. Boomers behave like Leakers. Mêlée only.

Tanks only. High spawn rate. Magnum and Molotov only.

Melee only. Tanks only. Tanks die with one hit. Tanks have different colors. Tanks multiply.

Infinite ammo. M16 and Magnum only. Laser sight. Health regeneration. Low health doesn't slow Survivors down. Tanks only. Dying Tanks split into two more.

No Common Infected. Dying Special Infected spawn a bunch of Uncommon Infected.

Start with Magnum. Startled Witches only. Witches don't incapacitate.

Extra Witches that are startled easily.

High spawn rate. Mêlée only. Healing is faster. Tanks die from one gunshot.

Spitters and Tanks only. High spawn rate. Infinite ammo. No reload. No incapacitations.

Smokers and Boomers only. Tanks have double health. Grenade Launcher and Magnum only. Infinite ammo.

Boomer and CEDA only. Bile attracts tanks. SMG only. Infinite ammo.

No Special Infected.

Common Infected are slow. High damage. No incapacitations. No ammo or health pickups.

Low gravity.

One shot kills. Magnum only. No reload.

Low Gravity. Hunters only. High spawn rate.

Special Infected immune to Pistols. Pistols only. Bile only.

Only features that didn't exist in L4D1. Tanks and witches still appear.

A combination of Hunting Party and Hard Eight.

???

Many hordes. Interactions are faster.

Tanks only. High spawn rate. Infinite ammo.

L4D1 Survivors are added as bots. Works for all official campaigns.

Witches only.

No rescues.

Realism. Common Infected only. Two incapacitations before death.

No rescues. No Common Infected.

Boomers only. One shot kills. High spawn rate.

Special Infected only. One shot kills. High spawn rate. No special attacks.

Jockeys only. One shot kills. High spawn rate.

Smokers only. One shot kills. High spawn rate.

Special Infected only. One shot kills. High spawn rate.

Spitters only. One shot kills. High spawn rate.

Tanks only. One shot kills. High spawn rate. Extremely high damage.

Special Infected only. One shot kills. High spawn rate. Random combinations of three.

Like Special Delivery, but with L4D1 features.

Survivors have 5 minutes to reach the SafeRoom or die. Boomers only. One shot kills. High spawn rate. Boomer Bile blindness only.

Survivors have 5 minutes to reach the SafeRoom or die. Chargers only. One shot kills. High spawn rate.

Survivors have 5 minutes to reach the SafeRoom or die. Hunters only. One shot kills. High spawn rate.

Survivors have 5 minutes to reach the SafeRoom or die. Jockeys only. One shot kills. High spawn rate.

Survivors have 5 minutes to reach the SafeRoom or die. Special Infected only. One shot kills. High spawn rate. Random combinations.

Survivors have 5 minutes to reach the SafeRoom or die. Smokers only. One shot kills. High spawn rate.

Survivors have 5 minutes to reach the SafeRoom or die. Special Infected only. One shot kills. High spawn rate.

Survivors have 5 minutes to reach the SafeRoom or die. Spitters only. One shot kills. High spawn rate.

Survivors have 5 minutes to reach the SafeRoom or die. Tanks only. One shot kills. High spawn rate.

Special Infected only. Bile Jar only. Infinite ammo. Bile Jar deals poison damage. No Common Infected.

Special Infected only. Pipe Bomb only. Infinite ammo. Short fuse.

Special Infected only. Molotov only. Infinite ammo. No Molotov damage.

Special Infected only. Randomized throwables only. Infinite ammo. No self or team damage. Bile Jar deals poison damage. Short fuse.

Low gravity. Pipe Bomb only. Infinite ammo. Short fuse.

Common Infected only. Molotov only. Infinite ammo. No fire damage. High damage. No incapacitations.

Like Triple Play, but with low gravity.

Like Bombing Run, but Hunters and Jockeys only. Larger explosions.

Low gravity. Molotov only. Infinite ammo. No fire damage. No incapacitations.

When incapacitated, Survivors start becoming infected after a set amount of time. The antidote is in the next Safe Room.

Being hit causes Bleed Out, and Adrenaline cures it.

If your temporary health drains to zero while infected you will die. Health kits don't remove the infection, they just delay it. The safe room holds the cure.

No weapon pickups. Killing Special Infected gives you the next weapon. Infinite ammo.

No weapon pickups. No Special Infected. Every 5 kills you get a new random weapon. Infinite ammo.

Weapons swtch every 10 seconds.

Weapons swtch every 10 seconds. Health regeneration. Special Infected only. Low gravity.

Weapons swtch every 10 seconds. Health regeneration. No incapacitations.

Weapons swtch every 10 seconds. Health regeneration. Hunters only. No special attacks.

Weapons swtch every 10 seconds. Health regeneration. Special Infected only.

Weapons swtch every 10 seconds. Health regeneration. Common Infected only. When Common Infected die, there is a chance to spawn a Special Infected in its place.

Weapons swtch every 10 seconds. Health regeneration. Many hordes. Interactions are faster.

Weapons swtch every 10 seconds. Health regeneration. Tanks only. High spawn rate.

Weapons swtch every 10 seconds. Health regeneration. Tanks only. Dying Tanks split into two more.

Weapons swtch every 10 seconds. Health regeneration. Common Infected are slow. Headshots only.

Weapons swtch every 10 seconds. Health regeneration. Iron Man. Special Infected only.

Weapons swtch every 10 seconds. Health regeneration. No Common Infected. Dying Special Infected spawn a bunch of Uncommon Infected.

Weapons swtch every 10 seconds. Health regeneration. Extra Witches that are startled easily.

Gib Fest. Laser sight. One hit incapacitations. Bleed Out after first incapacitation. Two incapacitations before death.

Gib Fest. Laser sight. No Common Infected. Dying Special Infected split into two more.

Gib Fest. No Common Infected. Special Infected die easily. High spawn rate. No incapacitations.

Gib Fest. One hit causes Bleed Out. Second hit causes death.

Bleed Out. Killing heals you. Two incapacitations before death.

Bleed Out. Killing heals you. Start with Magnum. Common Infected only. High damage.

Bleed Out. Killing heals you. Special Infected only.

Common Infected only.

No Common Infected. High spawn rate.

Half Special Infected.

Common Infected only. High spawn rate.

CEDA only.

Clowns only.

Workers only.

Mud Men only.

Fallen only.

Jimmy Gibbs only.

Riot only.

Uncommon Infected only.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Low Gravity. M60 only. Infinite ammo.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Boomers only. Some Boomers behave like Leakers. M60 only. Infinite ammo.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. When incapacitated, Survivors start becoming infected after a set amount of time. The antidote is in the next Safe Room.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. No rescues.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. No incapacitations.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Hunters only. High spawn rate. Hunters don't pounce.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Common Infected are slow. Headshots only. No incapacitaions. When health reaches 0, you have 2 minutes to reach the SafeRoom or die.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Gib Fest. No Common Infected. Special Infected die easily. High spawn rate. No incapacitations.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Common Infected only. When Common Infected die, there is a chance to spawn a Special Infected in its place.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. No Common Infected. Dying Special Infected split into two more.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Melee only. Tanks only. Tanks die with one hit. Tanks have different colors. Tanks multiply.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Infinite ammo. M16 and Magnum only. Laser sight. Health regeneration. Low health doesn't slow Survivors down. Tanks only. Dying Tanks split into two more.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Special Infected only. Randomized throwables only. Infinite ammo. No self or team damage. Bile Jar deals poison damage. Short fuse.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. No Common Infected. Dying Special Infected spawn a bunch of Uncommon Infected.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. If your temporary health drains to zero while infected you will die. Health kits don't remove the infection, they just delay it. The safe room holds the cure.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. Extra Witches that are startled easily.

One Survivor is randomly assigned as VIP and only has a Pistol. When they die, the whole team loses. High spawn rate. Mêlée only. Healing is faster. Tanks die from one gunshot.

Realism. Common Infected only. No HUD.

Realism. No HUD. No incapacitations.

Realism. Double Special Infected. No HUD.

Realism. M60 and Magnum only. Infinite ammo.

Common Infected are slow. One hit death. No HUD. No glow.

One slow Charger with low damage and high health. Three Boomers.

Realism.

Allows Survivors to play Scavenge against bot Infected.

No Common Infected. Double Special Infected.

Chargers and Spitters only.

Grenade Launcher and Magnum only. Infinite ammo.

No incapacitations.

No Special Infected.

M60 and Chainsaw only. Infinite ammo.

Realism.

Double Special Infected. No Common Infected.

Tanks only.

Uncommon Infected only.

L4D1 Survivors are added as bots. Works for all official campaigns.

One slow Charger with low damage and high health. Three Boomers.

Realism. One slow Charger with low damage and high health. Three Boomers.

No incapacitations.

Hunters only.

Only features that didn't exist in L4D1. Tanks and witches still appear.

Low gravity.

Mêlée only.

Double Special Infected. No Common Infected.

Low gravity. Lots of Tanks.

Smokers.

Gnome acts as bile.

Players can be Infected only.

Players can be Infected only. No healthpacks.

Players can be Infected only. No Tanks.

Players can be Infected only. Bleed Out.

Players can be Infected only. L4D1 Special Infected only.

Players can be Infected only. Boomers only.

Players can be Infected only. Chargers only.

Players can be Infected only. Hunters only.

Players can be Infected only. Jockeys only.

Players can be Infected only. Smokers only.

Players can be Infected only. Spitters only.

Players can be Infected only. Tanks only.

L4D2 maps feature L4D1 Survivors and vice versa.

Wave-based Survival, like Killing Floor.

Deathmatch. You start with a pistol. Each kill upgrades your weapon. First to kill someone with the 20th weapon wins.

Deathmatch. Magnum only. One shot kills.

Typical LMS. No respawn. Last player alive wins.

One player is randomly assigned to be Psycho. Psycho kills with one mêlée attack and gradually becomes invisible. Other players must kill Psycho to win, or die and lose.

Deathmatch. Molotov only. Infinite ammo. No self damage.

Mêlée only. No healthpacks. No pills.

Boomers only. No vomit cooldown.

Crouching makes you sprint.

Shotgun only. High ammo capacity. No incapacitations.

Map lighting is darker. Realism. Nametags appear.

Like Darkness Falls, but without HUD.

Chargers only. No charge cooldown. High spawn rate.

Jockeys only. High spawn rate. Jockeys have lower health and move faster.

Survivors have less resources.

Common Infected are silent.

Hunters and Smokers only. Sniper Rifles only.

Spitters only. High spawn rate.

No HUD. Headshots only. No glows. Pistols only. No incapacitations.

Like Darkness Falls, but with L4D1 features.

Realism. Common Infected only. High spawn rate. Fog.

Realism. Common Infected are silent and faster. Fog.

Realism. No sky. No flashlight.

Realism. Endless hordes. No incapacitations.

Realism. Darker map. Brighter flashlight. Limited resources.

Realism. Fewer Common Infected. No Special Infected. No HUD.

One slow Charger with low damage and high health. Three fast Spitters with weak spit.

No Common Infected. More health stuff.

Pistol only. Charger only.

No Common Infected. No Smokers.

Darkness. Hunters only.

Realism. No Special Infected.

Grenade Launcher and Magnum only. Infinite ammo. No reload. Special Infected only.

Fewer Infected. Pump-Action Shotguns and Grenade Launchers only. Limited resources.

Fewer Infected. Pump-Action Shotguns and Grenade Launchers only. Limited resources.

Fewer Infected. Limited resources.

Chargers only. High spawn rate. Instant incapacitation.

Pump-Action Shotguns and Pistols only. Limited health

Molotov only. Infinite ammo. No fire damage.

Jimmy Gibbs only.

One slow Charger with low damage and high health. Three fast Spitters with weak spit.

Death's Door. Bleed Out. Tanks.

Molotov only. Infinite ammo. No fire damage. No incapacitations.

No weapons.

2 vs 2 versus with AI controlled survivors.

Grenade Launcher only. Infinite ammo. Lots of Tanks.

• Rayman1103's Mutation Mod (Steam Workshop)
  
• Mutation Spreadsheet (Google Docs)[docs.google.com]
  
• Mutation Information (Facebook)[www.facebook.com]
  
• "Biohazard Zombie zone" by DarthByron[darthbyron.deviantart.com]