In The Winter's Tale, Shakespeare uses an extended metaphor to compare male mistrust of their wives to a disease. In Act 1, Scene 2, Camillo calls Leontes's growing distrust of his wife a "diseased opinion" and warns that it is "most dangerous." Later, he extends this metaphor when he tells Polixenes, "There is a sickness / Which puts some of us in distemper, but I cannot name the disease, and it is caught / Of you that yet are well." Camillo's language suggests that the disease of Leontes's mistrust is a contagious one originating in Polixenes, even though the latter is "yet...well" because he is not mistrustful himself.
In Act 2, Scene 1, Leontes's metaphor of a spider in his drink similarly identifies his distrust of his wife to the effects of consuming venom:
There may be in the cup
A spider steeped, and one may drink, depart,
And yet partake no venom, for his knowledge
Is not infected; but if one present
Th’ abhorred ingredient to his eye, make known How he hath drunk, he cracks his gorge, his sides,
With violent hefts. I have drunk, and seen the spider.
Leontes asserts that when a drink contains a venomous spider, one may only experience the effects of that venom once aware of the spider's presence. However, not only is this metaphor factually false—venom is harmful regardless of whether one is aware of its presence—but Leontes also errs in mistaking his unfounded suspicions for knowledge of the truth. He doesn't know that the spider is in his drink, that is, that his wife is unfaithful; he only thinks he does. In this way, Leontes's metaphor highlights the fallacies that distort his perceptions and lead him to engineer his own downfall. Indeed, Leontes ultimately refuses to accept a cure for his diseased opinion in the form of Paulina's "medicinal" words of truth, leading to tragedy. Camillo and Leontes's diverging uses of metaphor to characterize the "disease" of the latter's mistrust thus highlight the extent of Leontes's delusion.