Two references to disease or decay. How are they significant. explain
In Act 1.5.100, Marcellus makes his portentous remark, "Something is rotten in the state of Denmark." Of course, many things are "rotten" in the kingdom: the betrayal and murder of the king by his brother, Gertrude's infidelity and fickleness, and Prince Hamlet's denial of the throne, to name a few.
The theme of decay continues throughout the play. In Act 4.5.204-209, Ophelia compares the beauty and delicacy to the dying spring flowers to her own beloved fathers death:
There's fennel for you, and columbines.
There's rue for you, and here's some for me; we
may call it herb of grace o' Sundays. You(must) wear
your rue with a difference. There's a daisy. I would
give you some violets, but they withered all when
my father died. They say he made a good end.
The herbs Ophelia refers to here are not accidental or random. Fennel is known to be a favorite of snakes. They eat it when they shed their skins. Columbine was also known as a symbol of the Holy Spirit. And rue is probably chosen because of the word play, ie, to "rue" the day.