The Man Who Bit a Fell-Beast

[Boromir88]

Hmm..Aragorn believes that Merry was a victim of the Black Breath (in Bree) and yet seems to have escaped serious harm.

I agree that it seems to be a spiritual or psychological ill. As Merry was awoken from a disturbing dream, but I don't think suffered any other hurts from the Black Breath. Faramir's severe injuries come from multiple sources:

"Weariness, grief for his father's mood, a wound, and over all the Black Breath," said Aragorn.
...
"Walk no more in the shadows, but awake!" said Aragorn~The Houses of Healing

It's similar Theoden's words after being healed from Grima's crooked counsel: "Dark have been my dreams of late." Theoden's ailment was spiritual as well, words have meaning, and Grima's crooked counsel made him appear weak and sickly.

Perhaps, on its own, the Black Breath is not deadly? Merry is a resilient hobbit, who had his first brush with "sorcery" and he for the most part escaped harm; it gave him a bad dream. Faramir before riding out to war was troubled by Denethor's mood and as Aragorn remarks: "he had come close under the Shadow before ever he rode to battle.."

Edit: X'ed with Formendacil! Ahh this might be the first X'ed posting with a member outside of a werewolf game in years!