O Rose, thou art sick!
The invisible worm
That flies in the night
In the howling storm
Has found out thy bed
Of crimson joy
The word  “flies”  implies that her love is ‘in the air’ and is constantly on her mind and she is frightened of it becoming publicly known. 
 The word  “night”  could be seen to be when sexual intercourse usually takes place and therefore could be referring the sexual theme of the poem. Also it is dark at night and things can be easily hidden, so the secret affair between the two lovers is kept under lock and key whilst in the privacy of each other, at night.
The Sick Rose