At the end of act one scene four, as the ghost and Hamlet exit, officer Marcellus states that "Something is rotten in the state of Denmark (1.4.90)." They are starting to realize that things aren't right with the world they live in, and that more is on the way.
The final scene of act one has the ghost telling Hamlet how he died, and how it was from poison poured into his ear which spread throughout his body causing a scab to form over his body. He also gives Hamlet the idea for revenge against Claudius. Again, disease relates to this particular scene, as well as the whole first act in general. The final line of scene four summed up the chapter when Marcellus claimed something was in the air in Denmark. It provided a base foundation of disease, which affected everyone active person in that first act.
In the second act, Hamlet begins to go mad. "For if the sun breed maggots in a dead dog, being a god kissing carrion, - Have you a daughter (2.2.180-182)?" In one of Hamlet's speeches with Polonius, he uses that line. The speech in general is about how spoiled life is, as well as fully convincing Polonius of his insanity.
The third act is a very interesting one, with many events materializing during the scenes. A play called The Mousetrap was performed in front of the king and queen which reenacted the killing of king Hamlet. Hamlet used the play as a way of spreading his anger to Gertrude and Claudius because of their incestuous marriage, which he compared to a disease. Also during the play, the villain, Lucianus, uses the phrase "with Hecate's ban thrice blated, thrice infected (3.2.238)," which was describing the poisonous mixture in his hands. It serves as another form of symbolism for the marriage and the events that took place, because the poison not only infected the king, it infected every other part of the relationship.
After Claudius makes a hasty exit, retreats to privacy in order to pray.