Infernal Fever

Transmission & Vectors

Usually transmitted via a wound, it has been known in some cases to be airbourne when a desecrated item (of a fiendish nature) is burnt.

Causes

This disease affects beasts and humanoids, though its effects are more predominate seen in humanoids. Whenever a creature suffers an attack or cut from a devil, they must make a DC 15 constitution save or become infected.
If a mortal with infernal fever dies and it's soul is on the path to hell they will be permanently infected to spread the disease to other mortals when they visit the material planes.
If the mortal does not die while the infection is running it's course, then they are not at the risk of becoming infectious.

Symptoms

If caused by a wound, they get a fever, the infection site is black after an hour. 
If caused by inhalation of a burning desecrated item, they get the fever, black around their nostrils and the inside of their mouth. They develop a persistent cough.
After an hour they have trouble focusing and has disadvantage on Intelligence and Wisdom ability checks and saving throws.
If not treated after a day, the creature must make another saving throw against this disease. With a failure, the symptoms worsen and paranoia sets in. Whenever an infected creature enters a stressful situation, they become frightened, treating any non friendly creature as hostile and the source of its fear. Each additional day, the creature must make another saving throw. After three failures, the creature develops a permanent madness condition determined at random.

Treatment

Due to the supernatural origin of this disease, it is resilient to lesser magic. Any effect that would normally cure a disease instead allows you to attempt another saving throw with advantage instead. Only a Greater Restoration, Heal, or higher level spell can remove the effects. Otherwise, a flower known as angel’s feathers can be brewed into a tea by a creature proficient with herbalism (DC 16). One dose will prevent the disease from worsening, and the disease is cured after five doses.

Prognosis

Condition is rarely fatal but madness is a very typical result.

Hosts & Carriers

Fiends

Prevention

There is very little that can prevent this as it can even get around someone's natural immunity to disease and illness.

Type

Supernatural

Origin

Magical

Cycle

Chronic, Acquired

Rarity

Extremely Rare