Skill that increases when sick, or by making foes sick. Higher skill levels make you more resistant and raise intelligence and vitality.
The first part, "Skill that increases when sick," is self explanatory. If you become sick then your skill goes up.
The second part, "or by making foes sick," seems like a better option to me. You can make foes sick by using a weapon that can inflict colds.
If you know of any items that has that as an attribute, please comment below so I can add it. Thank you.