Various attacks cause ability score loss, either ability damage or ability drain. Points lost to ability damage return at the rate of 1 point per day (or double that if the character gets complete bed rest) to each damaged ability, and the spells lesser restoration and restoration offset ability damage as well. Ability drain, however, is permanent, though restoration can restore even those lost ability score points.
While any loss is debilitating, losing all points in an ability score can be devastating.
• Strength 0 means that the character cannot move at all. He lies helpless on the ground.
• Dexterity 0 means that the character cannot move at all. He stands motionless, rigid, and helpless.
• Constitution 0 means that the character is dead.
• Intelligence 0 means that the character cannot think and is unconscious in a coma-like stupor, helpless.
• Wisdom 0 means that the character is withdrawn into a deep sleep filled with nightmares, helpless.
• Charisma 0 means that the character is withdrawn into a catatonic, coma-like stupor, helpless.
Keeping track of negative ability score points is never necessary. A character's ability score can't drop below 0.
Having a score of 0 in an ability is different from having no ability score whatsoever.
Some spells or abilities impose an effective ability score reduction, which is different from ability score loss. Any such reduction disappears at the end of the spell's or ability's duration, and the ability score immediately returns to its former value.
If a character's Constitution score drops, then he loses 1 hit point per Hit Die for every point by which his Constitution modifier drops. A hit point score can't be reduced by Constitution damage or drain to less than 1 hit point per Hit Die.
The ability that some creatures have to drain ability scores is a supernatural one, requiring some sort of attack. Such creatures do not drain abilities from enemies when the enemies strike them, even with unarmed attacks or natural weapons.