Shivering Sickness

The insects native the Chult's jungles and marshes carry a disease know as Shivering Sickness. This disease can be prevented through the use of insect-repelling salve on all exposed skin. If this is not done, giants and humanoids who complete a long rest are exposed. While the salve works for all usual insects, giant insects also carry the disease. Any creature injured by a giant insect is exposed to the illness. 
An exposed giant or humanoid must succeed on a DC 11 Constitution saving throw or begin on stage 1 of the disease. Creatures with natural armor have advantage on the initial saving throw and grungs are immune to 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

Parasitic

Stage 1
The infected creature begins to feel unusually cold. This gives the creature vulnerability to cold damage.
Stage 2
The infected creature suffers from violent shivering and uncontrollable shaking, giving it disadvantage on attack roles and ability checks.
Stage 3
The infected creature begins to decline severely and if they do not improve their situation will become dire very quickly. They can no longer benefit from healing during a long rest and only regain half of the hit points from rolling hit dice. The creature also suffers 2 (1d4) cold damage every hour.