Sometimes humanoid enemies, particularly assassins, will poison you with their weapons. If you are poisoned by such a weapon, your attributes and health slowly decrease until the poisoning is cured. If you don't treat your condition in time, it is lethal.
When you have been poisoned, you occasionally receive a notification that "you feel somewhat bad." Your health decreases at a rate of 2 HP per in-game minute and any affected attributes by 1 point per in-game minute. Which attributes are affected seem to depend on which poison was applied to the weapon. Apparently, various types of poisons were defined during the game's development, as it is still evident in SPELLS.STD. However, in the final release only enemies are able to use them.
Curing a poisoning is only possible by consuming a Cure Poison potion, or casting a spell (circinate or self-made) with a Cure Poison effect. However, while the poison will no longer be actively harming you, these cures will not restore any attribute damage you have already suffered.
While Temple healers are not able to treat a poisoning, you can get your attributes restored at a Temple if you are a member. Speak to any NPC within the Temple, and you will be asked if your wish to have your attributes restored (provided your rank within the Temple is high enough to receive free healing). This attribute restoration feature is available as of patch v2.12 and later.
Alternatively, a custom spell with a "heal attribute" effect can be used, provided you have access to a Spell Maker and your Restoration skill and spell points are sufficient to cast it. There are no other methods, besides hacking the savegame, to restore damaged attributes.