The body and soul are so completely sick of each other that it's no surprise sickness surfaces as a heavyweight metaphor in the poem. The soul complains about actual physical pains, but then bends this into paradox, assuring us that the cure is even worse than the disease. The body, on the other hand, spins a web of metaphor, making the pains of emotion and memory vivid and understandable by comparing them to bodily diseases. What with the soul's seesaw between sick-and-well, and the body's psychological torment, it's hard to say whose pains are worse.
Lines 15-16: The body calls the soul a fever to emphasize both the discomfort and pointlessness of being alive. Instead of feeling grateful for the chance to be alive and go swimming and eat potato chips in the summer sun, the body thinks of life as a disease: unpleasant while it's happening and resulting in death.
Lines 31-32: By comparing emotional pain to sicknesses, the body translates the touchy-feely into stuff it already knows. At the same time, it underlines that every emotion—even joy and hope!—is painful and unwanted.
Line 33: Hope is compared to a cramp, a small nagging pain that grows the more you move that muscle. In the same way, hope has a tendency to grow the more you obsess over whatever you're hoping for.
Line 34: Fear is like a neurological disease because it's an overwhelming, systemic feeling. Think about the last time you read the actual ingredients in a packet of hot dogs and started shaking uncontrollably. Yep, that's fear.
Line 35: The body describes love as a pestilence, which means an aggressively infectious disease. Marvell was probably thinking of the plague—because nothing says love like swelling pus-filled buboes—but the metaphor holds for all diseases. Love makes you hot and cold, flushed and dizzy.
Line 36: It makes sense that hatred is compared to an ulcer, a sickness so hidden that it's often really hard to diagnose. Sure, some people are open about their enemies, but most of us like to keep that kind of gnawing, jealous dislike quiet.