Check: To heal poisons and diseases, the healer must make a successful check against their difficulty class. If he is successful, the target is healed and is cured a number of hit points equal to the skill roll, plus all modifiers. If the target suffers from no poisons or diseases, it is still cured of damage.
A negative result from the skill check will damage the target (provided the target is already damaged, diseased, or poisoned). However, such a result is rare since normally the heal skill will not be less than -1 (no ranks and a -1 wisdom modifier), and adding a die roll to -1 will not produce a negative result.