The men were not the only live things in the line. They were accompanied everywhere by their lice, which the professional delousers in rest positions behind the lines, with their steam vats for clothes and hot baths for troops, could do little to eliminate. The entry lousy in Eric Partridge's Dictionary of Slang and Unconventional English speaks volumes: "Contemptible; mean; filthy...Standard English till 20th C, when, especially after the Great War, colloquial and used as a mere pejorative." Lousy with, meaning full of, was "originally military" and entered the colloquial word-hoard around 1915: "That ridge is lousy with Fritz."
~ From The Great War and Modern Memory by Paul Fussell, p. 48