Blue mist fever

Blue mist fever, sometimes known as mad monkey fever, was a magical disease found in Chult as of the late 15 century DR.

Acquisition
The disease was spread by contact with odorless blue mists that would drift through the jungle. This mist could cover as much as at a time and could travel more than  in an hour. The mist was thin enough that it was not immediately obvious to travelers in the jungle as it approached. It was also known to linger in enclosed spaces, such as ruined buildings.

Effects
The disease attacked the minds of humanoids and giants. Those afflicted with blue mist fever began to see vivid hallucinations involving blue monkeys, which would set in within six hours of infection and would last at least a day, if not more. While not deadly in and of itself, intense bouts of the disease could put the victim in dangerous or deadly situations, especially in the middle of the jungle (e.g., from falling or drowning).

Prevention
Blue mist fever could be prevented by avoiding contact with the blue mists that spread it. As these mists were magical in nature, they could be destroyed using spells like dispel magic, although larger patches of the mist would not be negated in their entirety.

Cure
Afflicted individuals could fight off blue mist fever on their own. Eating the flesh of a yahcha beetle was known to help people snap out of their hallucinations, and antidotes for the disease were said to exist, although they were often in short supply. Otherwise, lesser restoration and similar curative magics were known to cure the disease.

History
At some point in the early-or-mid–15 century DR, a wizard and his shield guardian entered the Chultan jungle, but did not return after the wizard contracted blue mist fever and died after hitting his head on a rock. His shield guardian was left to sit unmoving in the forest, and came to be known as "Vorn" by the local batiri goblins of Yellyark.

Mist-borne diseases were a notable concern in the jungles of Chult by the 1470s DR, and blue mist fever was one of the better known Chultan diseases.

By the late 15 century DR, blue mists had settled into parts of the ruins of Omu. A yuan-ti pureblood named Sev became afflicted with it, and his brethren in the Fane of the Night Serpent imprisoned him for the duration of his sickness, which he spent throwing rocks at hallucinated blue monkeys.

Around the same time, blue mist fever was one of many diseases plaguing Camp Vengeance, the base of an Order of the Gauntlet expedition that had been undertaken to rid Chult of undead. The expeditionary leader, Niles Breakbone, sought to transport a number of his infected troops to Port Nyanzaru for recovery.