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
And his dark secret love
Does thy life destroy.
Does the dark secret love of the worm destroy the the rose?  The worm is viewed as the insidious, malevolent, engine of serpentine destruction in the poesy. 
 The clandestine romance is ruined — the crimson joy of the rose is ultimately destructive. . 
 We need to bear in mind that Blake believed in free love and was critical of the constraints of conventional morality. What he was criticising here was ‘the wrong sort of love’; exploitative, selfish, and hurtful to other parties.
The Sick Rose