The pervasive imagery in Henry IV, Part 2 is that of sickness, as applied to the state of England.
The note of a diseased kingdom is first struck in Act 1, scene 1, in which Morton relates how the Archbishop of York is joining the fight, telling his supporters that "he doth bestride a bleeding land, / Gasping for life under great Bolingbroke" (Bolingbroke is Henry IV).
The King takes up the same image of the kingdom as a sick body when he says to Warwick, "the body of our kingdom / How foul it is, what rank diseases grow, / And with what danger, near the heart of it" (Act 3, scene 1). In his reply, Warwick continues the image in a simile: "It is but as a body distemper'd, / Which to his former strength may be restor'd / With good advice and a little medicine."
The most extended treatment of the sickness image is made by the Archbishop, in his dialogue with Westmoreland in Act 4, scene 1: "we are all diseas'd," he says, and goes on to speak of a "burning fever" affecting everyone. His words, "And we must bleed for it," refers both to the blood spilled in battle and to the surgical practice, current at the time, of "bleeding" a patient to cure him. Continuing the medical metaphor, the Archbishop says that he comes to "purge th'obstructions which begin to stop / Our very veins of life."
The profusion of imagery of sickness emphasizes the disorder that prevails in England. In Richard II, the first in the cycle of plays that includes both Henry IV plays, the predominant image was of the kingdom as a garden that must be carefully tended. But following Henry's seizure of the throne from Richard, the kingdom (garden) has fallen into disorder. In Henry IV Part 2, the metaphor switches from an unruly garden to a sick body.
The sickness is also present in a literal sense. The old King is sick, as is Northumberland, who walks with the aid of a crutch (see Act I, scene 1). Younger, healthier men are needed if the kingdom is to recover its health.
Henry IV Part 2: Metaphor Analysis