The exact nature of the sickness is not clear, although it appears to me to be a sickness of attitude rather than a physical sickness. The 'invisible worm' could be both a phallic symbol and a symbol of corruption. The idea of a worm that 'flies in the night' is less problematic if it is understood that the word had a much broader denotation than we now allow it. In folklore, a dragon is sometimes referred to as a worm! However, it is not helpful to invest the worm in this poem with dragon-like qualities, as the image of corruption eating at the rose from the inside would be lost. That it flies 'in the night' suggests secrecy; the 'howling storm' is an image of violent discord in nature.