Debuffs are a unique gameplay element introduced in Spelunky 2. They can dramatically change gameplay for the player depending on when/where they get one.
Known debuffs include:
The player's heart after with poison effect applied
Poison: Inflicted by a variety of enemies . This debuff will make the player character appear to have a green tint, with poison bubbles emanating off of them sometimes. The player's health count at the top left will also have poison goop on it. Once the player is poisoned, they will take 1 damage about every 2-3 minutes or so. This effect can only be stopped by receiving health (from damsels or cooked turkeys)
Curse: Inflicted by fewer enemies but can be considered more dangerous than poison. This debuff leaves the player tinted dark grey, akin to stone. The health count will also turn dark grey. When applied, the debuff causes the player to immediately drop to 1 health. This can end a player's run quickly if received. The curse lasts until the player dies or until the player is rewarded by Kali via sacrifices (keep in mind this replaces the reward with a curse removal), so if the player is revived with the ankh, they will no longer be cursed. The curse debuff also allows the player to see ghosts, notably the Ghist shopkeeper in 1-4.
