When Leontes' jealousy erupts, he employs images of disease and poison. Railing against the immorality of women, he says, "Physic for't there's none" (Act 1, scene 2, line 200), i.e. it is like a disease for which there is no remedy. Leontes continues in the same speech, "many thousand on's, / Have the disease."
The poison image occurs when he compares himself to a man who has unwittingly drunk a spider that was in a cup. (It was believed that this was poisonous if the person was aware that he had swallowed the spider.) The images of disease and poison are appropriate for Leontes' jealous state of mind.
In the second part of the play, the dominant images change. Caroline Spurgeon comments in her book, Shakespeare's Imagery, that the imagery communicates a sense of "the common flow of life through all things, in nature and man alike the oneness of rhythm, of law of movement, in the human body and human emotions with the great fundamental rhythmical movements of nature herself" (p. 305). This is most noticeable in Act 4, in which images of nature dominate. It is as if the natural and human worlds come together in harmony, as well as the realm of the gods. This is of course appropriate, since the "disease" of Leontes' jealousy is being overcome by the natural tendency of life to regenerate itself. The nature images are especially apparent in the sheep-shearing scene (Act 4, scene 4), which shows a "green world" that is in marked contrast to the barren world of the first three acts. The images of nature that occur in Perdita's speeches show that she is completely aware of natural rhythms and their correspondence to human life. This can be seen for example in her distribution of flowers that are appropriate to the ages of the recipients (Act 4, scene 4, lines 103-28.