All diseases (except for vampirism and lycanthropy) lower speechcraft by 10% and movement speed by 5% each. Don't contract too many, or you'll have a hard time getting around.

Cure disease potions can no longer be crafted, only bought or found. 
They also have a higher value, so it costs more to purchase

Shrines no longer cure diseases, but have better effects on them

Ataxia
Lockpicking and pick-pocketing is 50% harder
Clubbed Hands: Unarmed attacks deal 10 additional points of damage
Ataxia is supposed to stiffen the hands up, so delicate things are harder to accomplish, but slugging things is easier

Black Heart Blight
Lowers carry weight by 50 points
Weakened Pulse: increases poison resistance by 20%
A weakened heart means it takes poison longer to take effect

Bone Break Fever
Stamina is reduced by 50 points
Fever Heat: Resist 10% of frost
Fevers make you warm, therefore ice isn't as effective

Brain Rot
Magicka is reduced by 50 points
Insanity: increases magic resistance by 5%
Brain Rot makes you go a little insane, so you ignore the effects of magic slightly

Droops
Lowers magic resistance by 35%
Depressing: Increases disease resistance by 20%
Being droopy and terrible makes disease less likely to find you hospitable? I don't know, this is a weird one

Rattles
Stamina regenerates 70% slower
Shiver: Resist 10% of fire
Rattling and shivering means you're cold, so fire would make you feel a little better

Rockjoint
Melee weapons are 35% weaker
Stiffened: Resist 10% of shock
Rockjoint makes you stiff like a lightning rod, so you resist shock? (I couldn't really think of anything else that would make sense. But wizards would
probably like this one)

Sanguinare Vampiris
Reduces health by 50 points

Witbane
Magicka regenerates 70% slower
Witless: Increases armor rating by 15 points
Witbane makes you oblivious, so you don't feel as much pain. Any non-wizard character might like this one

Vampires
The sun is much more intense on vampires:
Stage 1: sun reduces stats by 25
Stage 2: sun reduces stats by 50
Stage 3: sun reduces stats by 75
Stage 4: sun reduces stats by 100

To compensate, vampires get 3 more hours available outside. Originally 5am-7pm is daytime, now it's 7am-6pm


Shrines:

Akatosh: increase armor rating by 50 points

Arkay: increase health by 50 points

Azura: increase magic resistance by 25%

Dibella: increase speech by 35 points

Julianos: increase magicka by 50 points

Kynareth: increase stamina by 50 points

Mara: restoration spells cure 30% more

Mephala: immune to disease

Noctunral: fortify sneak by 30%

Stendarr: fortify block by 30%

Talos: unchanged

Zenithar: fortify speech by 30



In conclusion, this mod should fill your desire to bring more challenge to the game, because vanilla diseases are way too easy to overcome. If you're a devotee to Peryite or Namira, this should satisfy your gross desires.