The Poisoned debuff is marked by green droplets dripping out of the affected target. At the end of every round, the target takes Poison Damage until the effect expires. The amount of damage taken depends on the skill of the attacker.
Phenol Thoxian can inflict Poisoned when he is touched and when he attacks. Crafted Runes can grant similar passives: Poison Skin and Poison Touch.
Some enemies have passives that deal additional damage to Poisoned enemies such as Poison Bane or use abilties like Intensify Poison (3x damage to Poisoned targets.) Prevent your team from getting into a hairy situation by curing them with potions or cleansing abilities.
The following heroes have or can inflict/apply Poisoned:
Questionable Contrails (Special Attack): Ranged Light attack on all enemies that applies Burn, Poison, or Frostbite to each enemy, and buffs DEF of all allies. Unstable Bomb (Special Attack): Ranged Physical Attack on target enemy that applies up to 3 random status effects. Cooldown of this special attack is reduced for each status effect missed.
Toxic Form (Trait): Immune to Poison and Venom, Poisons on Basic Attacks and Poisons Melee attackers when hit. Removes corpses on killing blows. Slime Shot (Special Attack): Ranged Nature attack on all enemies with a chance to Poison, and Provokes the initial target.
Poison Rampage (Special Attack): Melee Physical attack on target and 2 random enemies with a chance to Poison. Fungus Amongus (Trait): Plants gain a chance to poison on basic attack per stack of growth.
Tinkerer's Tricks (Trait): All attacks have a chance to apply one of the following debuffs to each enemy hit: Chilled, Burned, Shocked, Diseased, or Poisoned. Only one of these debuffs will be applied per hit, and is applied to each enemy individually. This means that if you use Helmet Missiles, one enemy could get burned, another could be poisoned, and another might not be debuffed at all (resisted or immune).