At first glance, one might consider that the word "love" is being personified, but in fact the word "fever" really is the one that possess the movement associated with "longing." "Love" in this case is the disease, which is also the fever. The fever is longing to continue, or to nurse, the addiction. An object doesn't "long" for someone, something, or to satisfy addictions. Only people "long" to satisfy addictions; therefore, "longing" in this case refers to the fever's desire to live on within the disease. It's as if the fever, being a part of the disease, has its own personality and will to keep going. When a person "longs" for something, they fight to keep the situation going; or, they dream for relief from the suffering, whichever might be the case. These are all human characteristics which help to personify the idea of love as a fever which fights to continue on.