Pisanio is reading a letter from Posthumus revealing that he accuses Imogen of adultery. A liar has prevailed over Posthumus's "too ready hearing," and Imogen is "punish'd for her truth" (lines 6-7). Pisanio observes that Posthumus's mind, compared with Imogen, has sunk to the level of his fortune.
Posthumus asks Pisanio to murder Imogen. He shall have the opportunity when he delivers a letter to her. If to do so is to do good service, says Pisanio, then he shall not be counted a good servant; he will disobey the order and not tell Imogen of it.
Imogen enters and Pisanio gives her a letter from Posthumus, which she reads. He writes that he will brave her father's wrath in order to be restored by seeing her. He is at Milford Haven, and hints that she must follow her heart's prompting (presumably, to go there). Imogen is immediately excited by the thought of going to see Posthumus, and asks how long it will take to ride there. Pisanio is less enthusiastic, because he fears that she is riding to her destruction. But she insists that the only way forward for her is to go to Milford Haven and Posthumus.
Pisanio uses the imagery of disease ("strange infection") and poison to express Posthumus's unwarranted jealousy (lines 3, 5). But he makes clear that Posthumus's being a victim of such onslaughts does not absolve him of responsibility: Iachimo has simply "prevail'd / On thy too ready hearing" (lines 5-6).
Pisanio has followed Cornelius in disobeying his master for a virtuous purpose. Both Posthumus and the Queen have divorced themselves from truth, and in such cases, the true servant must prevent them from doing evil, if necessary by disobeying orders.
Cymbeline: Novel Summary: Act 3 Scene 2