The Plague Cloud is a mob that spawns during the boss fight called The Black Death. This mob will give you the Plague which makes you not able to regenerate health. Golden apples, regeneration potion, God apples and similar items will do nothing. Be careful to take these out quickly. Absorption or instant-health from golden apples will still help a little while fighting the boss with the plague.
Taming[]
Plague Clouds can be tamed by feeding them Cheese.