The Sick Rose

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.

William Blake 1757-1827

          La Rosa Enferma de William Blake

estás enferma, ¡oh rosa!
El gusano invisible,
que vuela, por la noche,
en el aullar del viento,

tu lecho descubrió
de alegría escarlata,
y su amor sombrío y secreto
consume tu vida.

Versión de Màrie Montand

So the question is: What is “the invisible worm that flies in the night”?

It is AUTUMN and its wind

We find here an allusion to male-female relationship. She is a rose, and has “the bed of crimson joy”. And he is “the invisible worm”, flying “in the night”. It’s interesting that he, “the worm”, destroys the “sick” rose’s life, and his love is “dark secret”. Our English Literature lecturer told us about this poem, and we were really surprised that the poem alluded to defloration…

Those English literature professors always have their mind in the gutter… William Blake is simply talking about blight that damages his beautiful roses:
Botrytis infections (a type of blight) are favored by cool, rainy spring and summer weather usually around 15C (60F). Gray mold can be particularly damaging when rainy, drizzly weather continues over several days.