This sickness, however, is not unto death, but for the glory of God, for in this sickness the soul swoons as to sin and as to all things that are not God, for the sake of God Himself, even as David testifies, saying: ‘My soul hath swooned away’. —that is, with respect to all things, for Thy salvation. For just as a sick man first of all loses his appetite and taste for all food, and his colour changes, so likewise in this degree of love the soul loses its taste and desire for all things and changes its colour and the other accidentals of its past life, like one in love. The soul falls not into this sickness if excess of heat be not communicated to it from above, even as is expressed in that verse of David which says: Pluviam voluntariam segregabis, Deus, haereditati tuae, et infirmata est, etc. This sickness and swooning to all things, which is the beginning and the first step on the road to God, we clearly described above, when we were speaking of the annihilation wherein the soul finds itself when it begins to climb this ladder of contemplative purgation, when it can find no pleasure, support, consolation or abiding-place in anything soever. Wherefore from this step it begins at once to climb to the second.
From CHAPTER XIX of Dark Night of the Soul by St. John of the Cross