Cackle Fever

This disease targets humanoids, although gnomes are strangely immune. While in the grips of this disease, victims frequently succumb to fits of mad laughter, giving the disease its common name and its morbid nickname: "the shrieks."

Symptoms manifest 1d4 hours after infection and include fever and disorientation. The infected creature gains one level of Exhaustion that can't be removed until the disease is cured.

Any event that causes the infected creature great stress—including entering combat, taking damage, experiencing fear, or having a nightmare—forces the creature to make a DC 13 Constitution saving throw.

On a failed save, the creature takes 5 (1d10) Psychic damage and becomes Incapacitated with mad laughter for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the mad laughter and the Incapacitated condition on a success. Any humanoid creature that starts its turn within 10 feet of an infected creature in the throes of mad laughter must succeed on a DC 10 Constitution saving throw or also become infected with the disease. Once a creature succeeds on this save, it is immune to the mad laughter of that particular infected creature for 24 hours.

At the end of each long rest, an infected creature can make a DC 13 Constitution saving throw. On a successful save, the DC for this save and for the save to avoid an attack of mad laughter drops by 1d6. When the saving throw DC drops to 0, the creature recovers from the disease. A creature that fails three of these saving throws gains a randomly determined form of Madness.