Update 11, increased the MaxStamina from 1/2/3% to 2/4/6%. Update 21 replaced the percentage based bonus with flat values, and changed the level requirements from 10, 20, and 40 to 25, 35, and 50. Prior to Update 21 it also gave resistance to Disease Damage.
In Update 29, immunity to the Poisoned status effect was removed, and Disease resistance was returned to this skill.