[William Cloud Hicklin]

There is no quotable proof of it, but I have wondered if "the Black Breath" that struck down so many who had fought the Enemy in LotR was symbolic of "shell shock" or "battle fatigue" in the old parlance, PTSD in today's terminology

Certainly this passage, referring not exactly to the Black Breath but the aura and cries of the Nazgul, was informed by what Tolkien observed in the trenches:

Out of sight and shot they flew, and yet were ever present, and their deadly voices rent the air. More unbearable they became, not less, at each new cry. At length even the stout hearted would fling themselves to the ground as the hidden menace passed over them, or they would stand, letting their weapons fall from nerveless hands while into their minds a blackness came, and they thought no more of war, but only of hiding and of crawling, and of death.