SARAH: The ghost describes the effect of the poison that Claudius had poured in his ear. Almost immediately, a tetter, or skin disease, covers the king's body. It forms a crust like the bark on a tree - that's the sense of the phrase "barked about."

RALPH: The ghost describes this strange skin disease as lazar-like. The reference here is to a biblical character - in the New Testament, in the Gospel of Luke, there is a parable about a beggar and a rich man. Lazarus, the beggar, is covered in sores, begs in vain at the gate of a rich man. Later, when they have both died, the rich man sees Lazarus comforted in Paradise, while he himself endures the agony of hell for his mean neglect of those in need.

SARAH: So, the ghost is directly comparing the effects of his poison to the leprous sores on Lazarus's body. But by referring to this parable, it seems that Shakespeare is also adding a sense of divine rewards and punishments to the ghost's story, that helps reinforce the context of the ghost returning from Purgatory.