This mod alters vanilla diseases to (1) not immediately show a message when you're infected, but become symptomatic some time later, and (2) gradually worsen over time if you don't find a way to cure them, through five stages of severity.  Chance of disease infection and progression is affected by Resist Disease effects; if you have 100% resistance, you won't contract any new diseases and any you currently have won't get worse.

If your character contracts a disease and doesn't cure it even though it reaches the final stage, it'll eventually begin to debuff your health over time.  If you continue to ignore it, and the debuff grows greater than your max health, you'll die.  This can be deactivated from the MCM by setting General > Health Reduction to 0 if you'd prefer.

MCM options are available to increase the difficulty of dealing with diseases if you want.  There's an option to have cure disease effects set the disease back one stage and prevent it from progressing for a while, but not fully cure it unless it's at the most mild first stage; severe infections will need multiple cure disease potions or effects, and you'll need to wait for each cure disease effect to finish working before you can apply another.  There's also an option for diseases which decrease magicka, stamina, carry-weight, or health to debuff them by a percentage (magicka reduced by 15%) instead of a flat value (magicka reduced by 25), to keep them threatening even to a high-level character.  Both of these options are off by default.

Chance of catching diseases and how quickly they progress can also be altered from the MCM.

Covered diseases:

• Ataxia
• Bone Break Fever
• Brain Rot
• Rattles
• Rockjoint*
• Witbane
• Black Heart Blight**
• Droops*
  

*In the vanilla game, Rockjoint and Droops appear to have nearly identical effects (decreasing one- and two-handed weapon damage).  I've chosen to alter Rockjoint to instead affect one-handed weapons and archery, and Droops to affect two-handed weapons and block, to differentiate them.
**This disease is not contractable in the vanilla game.  I've put together an optional file, requiring SPID, which adds a chance of catching this disease when in close proximity to Ash Spawn.

Sanguinare Vampiris is not changed in the main plugin.  Experimental edits to make this disease progress gradually are available in a separate plugin.  With it, reaching the final stage of the illness without curing it will begin the vanilla 3-day process of becoming a vampire.  However I've only done a rather cursory test of the functionality, as I never really play as a vampire myself, so please test it out alongside your other mods before using it on a save you care about!  (This is not especially likely to be compatible with vampire overhauls.)
Also, fair warning, with this add-on Sanguinare Vampiris debuffs your health pretty severely before you get to the point of actually becoming a vampire, and it takes a significant amount of in-game time (depending on your MCM settings).  I thought this felt thematically appropriate and that it actually added some interesting spice to the gameplay when I tested it, especially paired with needs mods that made it difficult to just wait it out in a safe location-- but if it doesn't match with your preferences or load order, best to be aware before installing it!  As an alternative, I'd recommend Subtlety of Sanguinare, which I think has a similar feel to the main mod but is easier to execute ingame if you want less nonsense in the way of getting to the vampire gameplay.

This mod was made to work alongside Nondescript Needs for my own game (but it's not required in any way).


Compatibility

Incompatible:

• Mods that use the 'DisDamage' magic effects for something are incompatible and would require a patch.  For this mod I decided to alter those effects to avoid needing to patch dialogue that checks for diseases.  Unfortunately this has the side effect that they now won't work properly if added to another disease, spell, or other effect added by a mod.  Let me know if you encounter such a case and I can look at a patch.
• Mods that alter diseases are probably incompatible.  You may be able to get things working by loading this mod after another that touches diseases, but I can't guarantee it; it depends on how the other mod makes its changes.
  

Partially compatible:

• Mods that add diseases are compatible if they don't use the DisDamage MGEFs, but this mod's progression system won't apply to them.  I can't guarantee I'll have time to make any consistency patches but you're welcome to ask.