Dire Diseases (5e Variant Rule)
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These rules are intended to make Curses and Diseases more dire.
All diseases and curses should be divided into "lesser", "greater", "incurable", and "soul bound".
Curing Diseases and Curses
Spells or effects that say they cure 1 disease now "cure 1 lesser disease". Spells that say they cure all diseases now "cure all lesser diseases or cure 1 greater disease". (The same rules apply to curses, except that the word disease should be replaced with curse).
No spell or effect in the game should be able to cure "incurable" or "soul bound" diseases and curses. Each should have their own special cure either stated in the description of set disease or curse, or made by the DM.
Incurable diseases and curses leave afflicted creatures upon death.
Soul bound diseases and curses stay with the afflicted forever, even if they die and come back with only very special ways of curing it. Some of them might even be incurable even by wish if the DM wills it.
How They Work
Most diseases are one greater sickness which is linked to, and causes a number of lesser sicknesses to effect the host. (Note incurable and soul bound can be greater or even lesser sickness which are just harder to cure).
An example of a sickness in 5e would be "Sewer Plague". The infected creature suffers one level of exhaustion, it regains only half the normal number of hit points from spending Hit Dice, and gains no Hit Points from finishing a long rest. At the end of each long rest, an infected creature must make a DC 11 Constitution saving throw. On a failed save, the creature gains one level of exhaustion. On a successful save, the creature's exhaustion level decreases by one. If a successful saving throw reduces the infected creature’s level of exhaustion below 1, the creature recovers from the disease.
With the Dire Diseases system, the "Sewer Plague" disease would look more like this:
Greater Sickness: "Sewer Plague". The next rest after catching Sewer Plague, the host is infected with the lesser sicknesses "Exhaustion", "Exhausting Sleep", "Halved Healing", and "Not Recovering Hit Dice". Sewer Plague does not cause any of these effects to stack, however when one or more is removed, it will re-infect the host with ONE of the effects that was cured at a rate that is dependent on how fast-acting the greater sickness is (this means that if all lesser sicknesses are cured, but the greater sickness remains, the host will regain all the lesser sicknesses day by day, one at a time. In addition, a greater sickness may never be cured until all related lesser sicknesses are cured).
Below are the effects of all of the lesser sicknesses listed under the "Sewer Plague greater sickness.
Lesser Sickness: "Exhaustion" This sickness causes the host to take on 1 level of exhaustion upon being infected by this sickness.
Lesser Sickness: "Exhausting Sleep" At the end of each Long Rest, an infected creature must make a DC 11 Constitution saving throw. On a failed save, the character gains one level of exhaustion. On a successful save, the character’s exhaustion level decreases by one level (note that this can only remove exhaustion levels caused by this sicknesses)
Lesser Sickness: "Halved Healing" When the infected creature would be healed, they are only healed half of the hit points that they would normally be healed.
Lesser Sickness: "Not Recovering Hit Dice" This sickness causes the host to be unable to recover hit dice upon finishing a long rest.
As you can see, a greater sickness on its own does not cause negative effects. Instead, each effect is separated and made into its own lesser sickness.
Most greater sicknesses will infect the host with all the lesser sicknesses and replace them when a lesser sickness is cured. The rate at witch this happens is based on how fast-acting the sickness is.
A Very Slow sickness will affect the host with one of its lesser sicknesses once every X number of weeks or months. I would suggest making sicknesses this slow incurable and or have very powerful lesser sicknesses)
A Slow sickness will affect the host with one of its lesser sicknesses once every X days.
An Average sickness will affect the host with between 2-4 of its lesser sicknesses after the first long rest taken after being infected by it. Then each day, it will either add another lesser sickness or replace 1 of its preexisting lesser sicknesses.
A Fast sickness will affect the host with all of its lesser sicknesses after the first long rest taken after being infected by it, then replaces 2 cured sicknesses each day.
A Deathly Fast sickness will affect the host with one of its lesser sicknesses every hour, and replaces cured lesser sicknesses at the same rate (feel free to change the rate to every 2 hours, 4 hours, d4 hours, or something like that)