This short, two quatrain poem by Blake is one of his most famous. It's dark and violent imagery is striking, and many people read the eight short lines and come away with in a dark mood.
First, the poem takes a widely recognized symbol of love, especially romantic love, the Rose, and shows its potential ugly side. According to the poem, the "dark secret" love is sick, infected, and related to death. This love is not the love that inspires the giving of flowers and Shakespearean sonnets, but the kind the "flies in the night."
One interpretation goes farther and claims the poem only speaks of lustful love and not any other lasting or pure love, and that it why it is loaded with such harsh diction and imagery.
"The Sick Rose" is a mere eight lines long and yet is a very famous poem. On a literal level, the poem says that a rose is sick because a flying worm, brought in by storm, has burrowed its way into the center of the flower and is killing it.
What makes the poem interesting is its openness to interpretation. Clearly, the poem is about more than an actual worm killing an actual rose. The question becomes, what do the rose and the worm symbolize? What you as a reader decide they represent will determine how you interpret the "internal," or metaphoric, meaning of the poem. Many critics have understood the crimson rose to stand for love between two people, given that a rose is a conventional symbol of romantic love, and the worm as some sort of secret anger or decay or even just a secret that is destroying the love the two once shared. Others say the rose stands for humans in general and that the worm represents the sin or evil in our souls that threatens to destroy us.