The theme of evil is also present as relates to illness/love. The final couplet gives the source of the "patient"'s sickness:
For I have sworn thee fair and thought thee bright
Who art as black as hell, as dark as night.
These analogies allude to the evil that is infecting the ability of the speaker to listen to his reason and, as the saying goes, "cure thyself." The speaker is so mesmerized by the object of his desire that he concedes that death is what awaits him:
Past cure I am, now reason is past care,
My thoughts and my discourse as madmen's are.
He describes his love almost as a demonic possession that has driven him mad and will soon claim his life. But, as a madman might, he clings stubbornly to his disease:
For that which longer nurseth the disease
Feeding on that which doth preserve the ill.
His "desire," he concludes "is death," and since he shows no signs of recovery, of giving his desire up, then he admits that the evil that is his love has won his life. This love that the speaker equates with evil is often noted to be the "Dark Lady" who figures prominently in the later sonnets.
In this sonnet, the poet describes the contrast between passion and reason in his love for the addressee. The poet says that his love has become a disease because he cannot act according to what his reason would suggest him. His unrestrained lust is making him behave like a madman. The sonnet contains an extended medical analogy as the poet's love is characterized as a "fever" and a "disease" in the first quatrain. Paradoxically, the poet finds himself wanting to preserve this disease and in the second quatrain the analogy continues by defing the poet's reason as a doctor who cannot convince his patient to cure his pathology.