and such heat makes the sufferer burn and suffer. However, unlike a typical disease, the patient (the speaker) in this case desires or wants the very germ or microorganism that causes or worsens ("nurses") his disease. In other words, he wants sex. But having sex only increases his lust, his desire. It only makes him worse.
The metaphor is continued or extended in the second quatrain. Reason is personified as a physician or doctor who walks out on his patient because the patient refuses to listen to or accept the doctor's help. To put it another way, the speaker refuses to listen to reason. So, the speaker then believes that the only possible cure to his condition, the only remedy that will bring an end to his desire, is death. His doctor (his reason), though, did not see death as an acceptable alternative (line 8). Thus, Shakespeare may be suggesting that death is not a reasonable or acceptable way to escape or avoid desire.
Some time has apparently passed for the patient as the reader begins the third quatrain. The patient or speaker is no longer thinking about death. The speaker just does not care. He no longer believes that he can find any cure for his disease. He no longer thinks that he can control his desire. Since his reason has left, the speaker is now mad. He cannot sleep at night (line 10) because his thoughts are always on his lust. He adds that both his thoughts and his speech (or "discourse") are those belonging to