Acid Plague

This plague is found in the Underdark and is transmitted by certain fungus spores that feasted on the remains of the most vicious and corrupted of oozes. 
These mushrooms grow to develop a tremorsense. They trigger when agitated or if a creature stays two consecutive turns within the reach of their tremorsense. 
When triggered, they release their spores in a radius centered on the fruiting body. These spores remain active for 1 minute or until dispersed by a strong wind. 
When a creature comes into contact with these spores, they must succeed on a Constitution save or contract the disease. After being infected, painful, red-brown sores develop all over their body within 24 hours, reeking of noxious acid and oozing black pus. These sores grow for 2 days before bursting, showering the creature in digestive, highly acidic fluid. Every round a creature is in contact with the acid, they take acid damage at the start of their turn. As an action, the creature can attempt a Dexterity saving throw, freeing itself from the acid on a success. 
The vile magic animating this plague knows if a creature is close to death and lets the spores erupt prematurely if a creature has less than half its hit points left. After erupting the sores recede somewhat, only to mature and burst again within two days. Every time the sores burst, the creature makes a constitution saving throw, ending the disease on a success. If a creature is killed by the acid, their body is decomposed into fertilizer and mushrooms spreading the acid plague will grow from their corpse.

Transmission & Vectors

Airborne fungal spores

Type

Fungal

Rarity

Common