Mad Monkey Fever

A magical mist creeps through the jungles of Chult seemingly without the aid of a breeze. Contact with this thin, blue, odorless mist can infect giants and humanoids with mad monkey fever. Thought to be created in some way by su-monsters, those who see into the mist often claim to see the shapes of animals and people within the mist. How this mist is generated is a mystery, and there are those in Port Nyanzaru and Fort Beluarian who would pay a good price for a controllable sample of the mist. 
A 20-foot square of mist can be dispelled by using dispel magic, but most areas of mist cover an area between 400 and 2,400 feet. This means that it will likely take more than one cast of the spell to entirely clear the mist, though a path might be able to be made more easily. 
A giant or humanoid who comes in contact with the mist must succeed on a DC 13 Constitution saving throw or begin on stage 1 of the disease. While infected, the creature must repeat the saving throw once every 24 hours. On a successful save, the creature's condition decreases a stage. If they fail the save, the condition progresses a stage. If the creature successfully saves while on stage 1, the disease is shaken off and they are no longer infected.

Type

Magical

Stage 1
The infected creature feels dizzy and a bit confused. This gives disadvantage on all Wisdom checks and saving throws. 
Stage 2
The infected creature begins experiencing vivid hallucinations. This gives the creature disadvantage on all and ability checks and saving throws. 
Stage 3
The infected creature is not sure of reality due to the overwhelming nature of the hallucinations. The creature is under the effects of the confusion spell.