Disease

     When searching for good diseases, you will come across rules like "every two hours roll a constitution save, if you fail you lose 1 wisdom and 3 dexterity, which you regain in 8 hours, unless you roll an odd number of saves, in which case save against Charisma or [...]", alongside names like "Blundecralian Anus Rot" or "Hairy Bones".

     Problems with these rules: many fiddly numbers, therefore easy to forget. Not intuitive; I like my diseases with names I immediately recognize. Inconsistent with the fiction of the world; if the non-player characters are afraid of a disease, it should be something fearful.
Worst of all these rules are lame. Dying horribly of a disease should be cool.

     Here is my take on disease. Originally for use with 5e, altered to be GLOG compatible-ish. Fairly generic.

• 1 – Cat Scratch.
  Vectors: Wild animal bites and scratches.
  Symptoms: Red lumps on the skin, then fatigue, then joint pain. Infected characters have two slots of exhaustion that rest does not remove.
  Cured by: A month of bed rest with plenty of fluids or a Lesser Restoration spell. Characters skilled in forestry or medicine may be able to craft a cure from wildflowers.
  Permanent effects: None. Characters who have once been infected can be infected again.

• 2 – Malaria
  Vectors: Jungles, swamps, other places with foul rotting things and stagnant water and biting insects.
  Symptoms: Chills, then fever, then sweating and shaking. Infected characters immediately drop anything in their hands when they roll a critical failure with any d20.
  Cured by: A month of bed rest with plenty of fluids or a Lesser Restoration spell. Characters skilled in forestry or medicine may be able to craft a cure from willow bark.
  Permanent effects: Characters who have once been infected with malaria will have advantage on saves against reinfection. Even after being cured, gaining exhaustion will give you the effects of malaria until you complete a long rest. Only a Greater Restoration or divine favor will end this permanent effect.

• 3 – Bubonic Plague
  Vectors: Filth, rats, exposure to places where people have died of Black Plague.
  Symptoms: Chills, then malaise, then high fever and muscle spasms, ending with death. At any point up to a month after your character comes into contact with a vector, your DM may require you to make a save against catching the Black Plague in this form.
  Cured by: A week of bed rest and plenty of fluids. Every day make a Constitution save and record the result. Three failures will result in instant death, three successes will cure you. A doctor present when you make the check or a casting of Lesser Restoration will immediately grant you one success. A casting of Restoration will cure you.
  Permanent effects: Characters who have once been infected will not be able to contract the Bubonic form again.

• 4 – Pneumonic Plague
  Vectors: Rats, infected persons, black magic and the undead.
  Symptoms: All Bubonic symptoms in addition to blood in the lungs. At any point up to a week after your character comes into contact with a vector, your DM may require you to make a save against catching the Black Plague in this form.
  Cured by: Three days of bed rest and plenty of fluids. Every eight hours make a Constitution save with a -2 penalty and record the result. Three failures will result in instant death, three successes will cure you. A skilled doctor or a casting of Lesser Restoration will immediately grant you one success, and a Greater Restoration will cure you.
  Permanent effects: Characters who have once been infected will not be able to contract the Bubonic or Pneumonic forms again. The horrible scarring of the necrotized flesh means you will be shunned by peasantry, but educated characters will know you are more dependable for having survived this, and will react accordingly.

• 5 – Septicemic Plague
  Vectors: Taking melee damage from infected persons or the undead, various black magics, infected material directly entering a wound.
  Symptoms: All Pneumonic symptoms in addition to blood from orifices and under the skin. Immediately upon exposure to a vector, your DM may require you to make a save against catching the Black Plague in this form.
  Cured by: Nothing. Every 1 minute make a Constitution save with a -2 penalty and record the result. . Three failures will result in instant death, three successes resets both counters. Every time you reach three successes, you have a 1-in-6 chance of recovering. A casting of Restoration will immediately grant you one success, and a Greater Restoration will cure you. Doctors will be of no help.
  Permanent Effects: Characters who have once been infected are now immune to the Black Plague, and furthermore take half damage from evil magic. You must wear a mask to go about society, and removing that mask may incite a Morale check from all non-player characters present.

• 6 – Leprosy
  Vectors: Dark magic is the most common vector, as it is difficult to catch this disease from lepers.
  Symptoms: Loss of sensation in the extremities accompanied with discoloration of skin. Lepers have disadvantage on saves against all other diseases, but have advantage on saves against Fear and are unaffected by pain.
  Cured by: Only divine favor can cure leprosy.
  Permanent Effects: The disease itself is permanent.

• 7 – Rabies
  Vectors: Wild animal bites and scratches, damage from the undead, damage from infected persons.
  Symptoms: Painful thirst and fear of water, followed by loss of speech and inability to understand any language, ending with death. Rabies develops rapidly over the course of 1d4+1 days.
  Cured by: Rabies can not be cured by normal means and almost always kills the infected person. Cleaning the wound carefully within an hour from exposure to a vector will ensure the disease is not caught at all, but once a character has been infected only a Restoration spell or better will save them.
  Permanent Effects: None. Characters who have once been infected by rabies can be infected again.

• 8 – Red Death
  Vectors: Dark magic, skin contact with blood of the infected, the undead.
  Symptoms: Confusion and disorientation, then blood from all orifices, ending with death. One day after coming into contact with a vector, you have a 5-in-6 chance of contracting the Red Death. The disease develops rapidly over the course of a few minutes.
  Cured by: Only Greater Regeneration or divine favor can save an infected person.
  Permanent Effects: The corpse of the infected person is damaged to the point that they cannot be brought back from the dead without a True Resurrection or the direct intervention of the being that possesses their soul (usually their deity).

• 9 – Scurvy
  Vectors: Extended use of rations in the place of fresh food (~one month, depending on the rations).
  Symptoms: Joint pain, then opening of old wounds and tooth loss. Characters suffering from scurvy do not heal when taking a Lunch, and healing spells are only half as effective.
  Cured by: While any healing spell can hold off the effects for one day, scurvy is a nutritional deficit and will quickly return. It can easily be cured by eating fresh fruit and vegetables for a few days.
  Permanent effects: None. Characters who have once been infected can be infected again.

• 10 – Tetanus
  Vectors: Dirty or rusty weapons, such as those carried by monsters.
  Symptoms: Bone-breaking muscle spasms and difficulty speaking. Infected characters cannot speak, and can not cast any spells which require them to speak. Any time the character fails a Strength check, they take 1d4 damage as their muscles lock and tear.
  Cured by: A month of bed rest with plenty of fluids or a casting of the Lesser Restoration spell. Characters skilled in forestry or medicine may be able to craft a cure from charcoal and alcohol.
  Permanent effects: None. Characters who have once been infected can be infected again.

• 11 – Lyme
  Vectors: Insects common in temperate forests.
  Symptoms: Joint pain, large bullseye rashes, then delirium. Infected characters are partially paralyzed. Each morning roll a d4:
  

  1. Face paralyzed, cannot speak.
  2. Legs paralyzed, must crawl at half speed or be carried.
  3. Arms paralyzed, cannot use their hands.
  4. Lungs paralyzed, unable to do anything except lie on their back and focus on breathing. Every time they are forced to roll a d20 they receive two slots of exhaustion.

  Cured by: A month of bed rest with plenty of fluids or a casting of the Lesser Restoration spell. There is no normal cure.
  Permanent effects: Even after being cured, gaining exhaustion will give you the effects of Lyme until you complete a long rest. Only a Greater Restoration or divine favor will end this permanent effect.

  
  

• 12 – Common Cold
  Vectors: Swimming, other people, being cold.
  Symptoms: Mild headache and a runny nose.
  Cured by: This disease respects nothing, neither of Man nor G_d. There is no medicine, and no magic, which can cure the common cold. Only 1d3+1 days will save you.
  Permanent effects: None. Characters who have once been infected can be infected again.

Conclusion:

     Lyme, Tetanus, Scurvy, Rabies, Malaria and Cat Scratch are worksite hazards for adventurers. Note that only Rabies has the primary effect of killing the character. The other diseases are intended to complicate gameplay, though you can certainly have them be lethal if left untreated.
     The three types of Bubonic Plague, along with the Red Death, are nation-destabilizing metathreats. These might be brought by an angry g_d, or they could be part of the plan of an ancient lich.