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DISEASE
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From DMG p. 257...

Basics...
* Contagion: DC to avoid disease when contacted.
* Incubation: Time for symptoms to appear, typically 1d4 hours or 1d4 days.
* Symptoms: How disease manifests.
* Effect: Game effects on character.
* Recovery: Saving throw DC needed after each long rest to recover, and number of recovery rolls needed to be cured. (Typically a Lesser Restoration spell automatically cures one disease.)

HOUSE RULE: A Hard (DC 20) skill check of either Wisdom (Medicine) or Wisdom (Herbalism kit) allows one creature to grant advantage on next Constitution saving throw against a disease. Only one such skill check is allowed to aid a specific creature per day, but a character may attempt to treat several patients (a number equal to the healer's Intelligence modifier, at least 1 per day).

Examples of diseases...

* Sewer Plague: Contact with vermin or filth, DC 11 Constitution saving throw, fail and 1d4 days later suffer fatigue and cramps. Suffer one level of exhaustion, only recover half hit points from Hit Dice in a short rest, no hit points recovered from a long rest. After each long rest, make DC 11 Constitution saving throw. Failure adds one level of exhaustion, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends.

* Common Cold: Contact with infected humans, DC 11 Constitution saving throw, fail and 1d4 days later suffer fatigue and excess mucus in nose and down throat. Suffer one level of exhaustion. After each long rest, make DC 6 Constitution saving throw. Failure adds one level of exhaustion, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends.

* Common Flu: As for common cold, but symptoms add fever, chills, cough, body aches, and recovery is a DC 11 Constitution saving throw.

* Malaria: Contact by bites from infected mosquitos, DC 14 Constitution saving throw, fail and 1d10+5 days later suffer fever, vomiting, headaches, and later yellow skin and possibly seizures. Suffer one level of exhaustion and take 1d4 poison damage. After each long rest, make DC 11 Constitution saving throw. Failure adds one level of exhaustion, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends.

* Ebola Virus: Contact with blood or fluids of infected mammals (mainly humans), DC 11 Constitution saving throw, fail and 2d10 days later suffer fever, then vomiting and possibly excessive bleeding. Suffer two levels of exhaustion at onset. After each long rest, make DC 13 Constitution saving throw; suffer disadvantage on save if the infected cannot drink a normal or better daily intake of water or similar fluids. Failure adds one level of exhaustion, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends and the survivor cannot be infected by the disease again for 10 years. 

* Rabies: Contact by bites from infected mammals, DC 12 Constitution saving throw, fail and 2d6 weeks later suffer difficulty swallowing and hydrophobia (fear of water), then foaming at the mouth and finally aggressively violent behavior (biting). Suffer one level of exhaustion. After each long rest, make DC 25 Constitution saving throw. Failure adds one level of exhaustion, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends.

* Smallpox: Contact with infected humans, DC 16 Constitution saving throw, fail and 2d10 days later suffer gradually worsening skin rashes, fever, headaches, skin lesions, pustules, and finally flakey scabs. Suffer one level of exhaustion and take 1d4 poison damage. After each long rest, make DC 20 Constitution saving throw. Failure adds one level of exhaustion, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends. Those who gained up to level 4 or 5 during the course of this disease permanently lose 2 Charisma points due to horrific scarring all over the creature's body.

HOUSE RULE: Generic diseases...

* Generic Disease (level 1 to 4): Contact with infected creatures or area, DC 10 Constitution saving throw, fail and 1d4 days later develop symptoms. Suffer one level of exhaustion. After each long rest, make DC 5 Constitution saving throw. Failure adds one level of exhaustion, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends.

* Generic Disease (level 5 to 10): Contact with infected creatures or area, DC 15 Constitution saving throw, fail and 1d4 days later develop symptoms. Suffer one level of exhaustion and 1d6 poison damage. After each long rest, make DC 10 Constitution saving throw. Failure adds one level of exhaustion and causes 1d6 poison damage, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends.

* Generic Disease (level 11 to 16): Contact with infected creatures or area, DC 20 Constitution saving throw, fail and 1d4 days later develop symptoms. Suffer two levels of exhaustion and 2d6 poison damage. After each long rest, make DC 15 Constitution saving throw. Failure adds one level of exhaustion and causes 2d6 poison damage, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends.

* Generic Disease (level 17 to 20): Contact with infected creatures or area, DC 25 Constitution saving throw, fail and 1d4 days later develop symptoms. Suffer two levels of exhaustion and 3d6 poison damage. After each long rest, make DC 20 Constitution saving throw. Failure adds one level of exhaustion and causes 3d6 poison damage, success removes one level of exhaustion. When last level of exhaustion is removed, disease ends.

Variations...

* Wasting Disease: Make Strength saving throw instead of Constitution, and poison damage (if any) is instead necrotic damage.
* Brain Disease: Make Wisdom saving throw instead of Constitution, and poison damage (if any) is instead psychic damage.

Undead diseases...

* Mummy Rot: Contact by touching (or hit from) an undead mummy, DC 12 Constitution saving throw, fail and suffer a curse. Cannot regain hit points by any means and hit point maximum is reduced by 10 (3d6) for every 24 hours after infection. When hit points reach 0, body turns into dust. Disease cannot be removed by the Lesser Restoration spell, but a Remove Curse, Greater Restoration or Wish spell is immediately effective.

* Romero Zombie Virus: Contact by bite from infected undead, DC 12 Constitution saving throw, fail and 1d4 hours later suffer one level of exhaustion and take 1d4 necrotic damage. Cannot regain hit points by any means and hit point maximum is reduced by 7 (2d6) for every hour after infection. When hit points reach 0, body dies for 1d4 minutes and then reanimates as a ghoul.