What is the disease of the human spirit?

Do spirits suffer by contact with mortals?

Oh no, but sometimes they are made better by such contact. Mortals may, and do suffer by contact with certain spirits of a low or evil nature, that is, when they do not exert the power, which all men possess to drive back these evil ones.

There are some of these too, who, when they come to rise from their debased state to a higher will feel all the deeper remorse for the deeds they have perpetrated in contact with humankind.

—Hafed, Prince of Persia