Act I Horatio says that the moon "Was sick almost to doomsday with eclipse."
The ghost uses words in Act I associated with disease to describe his poisoning and death:
The leperous distillment, whose effect
Holds such an enmity with blood of man
That swift as quicksilver it courses through
The natural gates and alleys of the body,
And with a sudden vigor it doth posset
And curd, like eager droppings into milk,
The thin and wholesome blood. So did it mine,
And a most instant tetter barked about
Most lazarlike with vile and loathsome crust
All my smooth body.
The motif of disease and decay creates and enhances the atmosphere of the play. Marcellus' line in Act I sums it up very well: "Something is rotten in the state of Denmark."