NOUSON could not appear so often in poetry and in such a variety of circumstances that we could imagine a traditional device by which it and the epithet modifying it could be placed in the line in the position in which we see them here; nor can we suppose that the essential idea 'he sent a plague throughout the camp' would be so common that the bards would have created a traditional formula to express it in the space contained between the beginning of the line and the hephthemimeral caesura. The only possible reason for the presence of KAKHN in this line is the particular desire of the poet to introduce this word into his sentence; and therefore we must recognize that he wanted to say, not to be sure that the plague in question was worse than other plagues, but that at this time it was bad for the Achaeans.
The only other resource I have at hand is Thomas Seymour's note from Perseus, which seems to agree (?):
kakên: the adj. is explained by the following (paratactic consecutive) clause, the first word of which takes up the thought of the adj.
I always understood KAKHN to indicate that it was a particularly terrible plague (for no reasoned reason). But it seems possible to me that KAKHN could indicate both that it was a particularly terrible plague and that it was bad for the Achaeans at this time. Any ideas?