* A medical skill (or doctors in towns) may fix critical damage (ala FO crippled limbs) and stop disease (OB stats and FO addictive visuals) progressing to new levels.
* Once a disease is stopped, you still need to heal or rest.
* Restoration magic can only heal normal damage, not critical damage or cure diseases (or it defies the purpose of having increased impact in the first place).
* Magic (and potions) may *temporarily* suppress the symptoms though.
* Left untreated, all (except one/two) diseases will fix themselves given a very long time of rest, but your character will be basically unplayable.
If magic and alchemy is allowed to "do anything", they become overpowered and the above serves no point in having in the game at all.
snip Letting skills give you any ability to cure things makes them irrlevant?
Alchemical and Magical cures should be options, but not at novice levels, and you would need to actually know what specific set of ingredients to use to make specific cures with alchemy, rather than a single, universal "Cure Disease". You would have to have a good level at restoration to actually cure diseases, but you should be able to use spells that temporarily bring lowered stats back up to normal, so if you have a serious disease but are in the middle of nowhere, you could use such spells to keep yourself capable of fighting, for a short time at least.
This isn't fallout, where you have to rely on others for everything until the end of the game.