Poison!

Overview
This is an ability applied on contact with an enemy. Meaning that when an entity from a poisonous unit hits a single enemy from a unit, they receive this debuff. This makes poisonous units useful for the front line as they can poison what they are fighting against and may also affect enemies they are next to if the fighting is close and intermingled with other enemy units.

Units with this ability
Greenskins
 * Night Goblins
 * Night Goblins (Fanatics)
 * Night Goblin Archers
 * Night Goblin Archers (Fanatics)
 * Night Goblin Warboss
 * Forest Goblin Spider Riders
 * Forest Goblin Spider Rider Archers
 * Night Goblin Squig Hoppers
 * Skarsnik
 * Giant Spider (Mount)
 * Catchweb Spidershrine (Mount)
 * Da Rusty Arrers
 * The Arachnarok Queen
 * Spider Hatchlings
 * Deff Creepers
 * Da Eight Peak Loonies
 * Durkit's Squigs
 * Da Swamp Fings (River Trolls)
 * Feral Wyvern

Vampire Counts
 * Crypt Ghouls
 * Crypt Horrors
 * All units in an army led by Helman Ghorst
 * The Feasters in the Dusk
 * The Konigstein Stalkers

Warriors of Chaos Beastmen
 * Chaos Warhounds (Poison)
 * Morghur
 * Chaos Warhounds (Poison)
 * Chaos Spawn (Beastmen)

Bretonnia


 * Wardens of Montfort (Mounted Yeoman Archers)
 * Peasant Bowmen (Pox Arrows)

Wood Elves
 * Glade Guard (Hagbane Tips)
 * Glade Riders (Hagbane Tips)
 * Giant Spiders
 * Forest Dragon
 * Sisters of Twilight

Dark Elves
 * Any Dreadlord with the Poisonblade or Serpentheart Names of Power
 * Slaanesh's Harvesters (Doomfire Warlocks)
 * Khainite Assassin
 * Death Hag
 * Ravagers of Rakarth (Scourgerunner Chariot)

Lizardmen
 * Stegadon and Blessed Stegadon (ranged attacks)
 * Skink Skirmishers and Blessed Skink Skirmishers (ranged attacks)
 * Chameleon Skinks and Blessed Chameleon Skinks (ranged attacks)
 * Skink Cohort (Javelins) (ranged attacks)
 * Skink Chief (ranged attacks)
 * Terradon Riders
 * Pahaux Sentinels (Terradon Riders)
 * Red Crested Skinks
 * Red Crested Skink Chief
 * Bastiladon (Ark of Sotek) (ability)
 * Dread Saurian (ranged attacks)
 * Shredder of Lustria (Dread Saurian)
 * Amaxon Barbs

Tomb Kings Skaven
 * High Queen Khalida (she also gives poison attacks to her entire army)
 * Necropolis Knights (Halberds)
 * Necropolis Knights
 * King Nekhesh's Scorpion Legion (Skeleton Spearmen)
 * Venom Knights of Asaph
 * Gutter Runners (Poison)
 * Gutter Runner Slingers (Poison)
 * Master Assassin
 * Brood Horror
 * Brood Horror (mount)

Items which enable poison attacks
Items:

Mortuary Cult
 * Fang of Qu'aph
 * Ouroborous

Legendary:
 * Cobra Mace of Mazdamundi
 * Blade of Nurglitch

Other:
 * Blade of Corruption
 * Emerald Collar
 * Warlock-Augmented Weapon
 * Venom of the Firefly Frog
 * Everbleed

Spells

 * Overcast Pestilent Breath from Skaven Spells of Plague

Character traits
See character traits

Followers which enable poison attacks
Followers:

TWW1 TWW2
 * Pox Carrier
 * Herbalist
 * Carnivorous Horticulturist
 * Alchemist

Trivia
This is the most common Contact Effect in the game.