As well as Faramir's case being the most urgent, a more pragmatic reason may be that Aragorn has immediate need of Faramir. Faramir is now the Steward of Gondor, and his ready allegiance is of great value.

“Anyway, a difference in the use of ‘magic’ in this story is that it is not to be come by by ‘lore’ or spells; but is in an inherent power not possessed or attainable by Men as such. Aragorn’s ‘healing’ might be regarded as ‘magical’, or at least a blend of magic with pharmacy and ‘hypnotic’ processes. But it is (in theory) reported by hobbits who have very little notions of philosophy and science; while A. is not a pure ‘Man’, but at long remove one of the ‘children of Lúthien’.”

Letters 1e then adds a footnote: "Alongside the final paragraph, Tolkien has written: ‘But the Númenóreans used “spells” in making swords?"

Gas! GAS! Quick, boys!—An ecstasy of fumbling
Fitting the clumsy helmets just in time,
But someone still was yelling out and stumbling
And flound’ring like a man in fire or lime.—
Dim through the misty panes and thick green light,
As under a green sea, I saw him drowning.

Extract from Dulce et Decorum Est, a poem by Wilfred Owen, describing the grim and gruesome reality of First World War battle (in this case being under poison gas attack). This reality is presented as contrast to idealised patriotism of propaganda of the period - the Latin phrase of the title is one that Tolkien and his school class (and all the other, similarly straight outah school young officers) would have been instantly expected to understand, and identify as Horace: “It is sweet and fitting to die for one’s country.”

Uncontrolled anxiety during a gas attack could cause men to tear off their protective masks, or act ‘as though they had temporarily lost their reason’. Later in the war Charles Wilson, a regimental medical officer with the Royal Fusiliers, argued that mustard gas had ‘partly usurped the role of high explosive in bringing to a head a natural unfitness for war, or less commonly in undermining fitness sapped by exceptional stress in the field’.

[from the Conclusions section] ...While poets such as Wilfred Owen emphasized the trauma of soldiers dying from gas, their suffering was not significantly different from a terminal stomach wound or shrapnel damage to the head and face. This raises the question whether gas had a particular capacity to inspire terror, or whether the initial novelty and the continual refinement of toxins and delivery systems were responsible for its enduring psychological impact. During the period before the issue of effective respirators, Charles Cruttwell, an infantry officer, believed that gas undermined a basic survival mechanism. A serviceman subjected to artillery bombardment had few, if any, defensive options, and trusted to luck. However, when he was exposed to cloud gas, Cruttwell argued, it was impossible to evoke the protection of chance – ‘if the very air which he breathes is poison, his chance is gone: he is merely a destined victim for the slaughter’. By contrast, shrapnel was tangible. It could be removed from a wounded soldier’s body by a surgical procedure, but no physician could decontaminate a man’s lungs, and it was popularly believed that, once toxins had been metabolized, the respiratory system remained damaged for ever.

...In addition to the deliberate exploitation of surprise and uncertainty, fears evoked by gas owed much to broad cultural themes. Some toxic chemicals, like phosgene, could not be readily detected through the senses and triggered powerful vestigial fears of mysterious threatening forces. They touched on a deep human concern about the risk of being invaded by a potent and unseen force. Chemical weapons were unfamiliar, which created opportunity for rumour and exaggeration. Beliefs about gas often inspired strong emotions that could disrupt the rational evaluation of evidence and the formation of coping mechanisms. Fears may have been intensified because gas was a product of science and cutting-edge technology. Man-made disasters have generally been experienced as more troubling than natural ones.

Terror Weapons: The British Experience of Gas and Its Treatment in the First World War by Edgar Jones Author manuscript available here Published in final edited form as: War Hist. 2014 Jul; 21(3): 355–375.doi: 10.1177/0968344513510248

So passed the sword of the Barrow-downs, work of Westernesse. But glad would he have been to know its fate who wrought it slowly long ago in the North-kingdom when the Dúnedain were young, and chief among their foes was the dread realm of Angmar and its sorcerer king. No other blade, not though mightier hands had wielded it, would have dealt that foe a wound so bitter, cleaving the undead flesh, breaking the spell that knit his unseen sinews to his will.

1. What is it about the Nazgul that they are able to have such a pernicious effect on human beings without even making physical contact? It can’t just be the overwhelming fear they engender, because once away from them that should have no lasting physical effects—other than PTSD or something like that. I suppose I’m saying “what is the black breath”?

Aragorn enters the houses of healing “cloaked in grey”; and at least earlier when meeting Imrahil, he was likely hooded from what seems to be implied in that section. But although he is hiding his identity in some respects and underplaying it in others, it’s as the king that he attempts his healing mission, even stating that his healing abilities were “given to him,” possibly partly as a member of the “line” of which Elrond was the “eldest” member, but as the chapter goes on, it also seems to be tied inextricably to his kingship.

1. Or may be at this point in the history, it’s the same thing?

2. Aragorn says that he is “Envinyatar, the renewer.” Is it possible that the people Aragorn heals are also being renewed in some way?

1. Again we have the Nazgul-effect, operating on Faramir over a long period of time so that even a man of “staunch will” eventually succumbs. What does it say about Faramir that he was able to resist so long under such harrowing conditions?

2. How did Aragorn come to know of Denethor’s “mood”?

1. Where was Faramir, do you think?

1. Where would Faramir have obtained this knowledge; and why love? What could have transpired in that “dark vale” to bring about what almost seems like a deep and established friendship? Perhaps partly the fact that Aragorn had come to his rescue, pursuing him, literally, into the valley of the shadow of death?

I can't imagine that the gas attacks were completely irrelevant in Tolkien's Black Breath idea, even if it's just sort of a generalized, vague idea in the background. This: ". . . cause men to tear off their protective masks, or act ‘as though they had temporarily lost their reason,'" is very close to what Tolkien write about the Gondor rout: “flying wild and witless here and there, flinging away their weapons, crying out in fear, falling to the ground.”

And Túrin being under the spell of Glaurung hearkened to his words, and he saw himself as in a mirror misshapen by malice, and loathed that which he saw.

Then suddenly Glaurung withdrew his glance, and waited; and Túrin stirred slowly, as one waking from a hideous dream.

Her [Nienor’s] will strove with him for a while, but he [Glaurung] put forth his power, and having learned who she was he constrained her to gaze into his eyes, and he laid a spell of utter darkness and forgetfulness upon her…

Then he drew her eyes unto his, and her will swooned. And it seemed to her that the sun sickened and all became dim about her; and slowly a darkness drew down on her and in that darkness there was emptiness; she knew nothing, and heard nothing, and remembered nothing.

Maybe, similarly, any elf can run lightly on snow but nobody else can. And maybe only Aragorn individually (or only some group of his relatives) can do what Aragorn does with the Athelas here. Of course anyone tryng to make a Venn diagram of who can uniquely do what will quickly find they have too little information to be sure the chart is right!

What is the black breath? To me it seems more explicitly developed in Harry Potter where the dementors suck the life out of people, or at a minimum drain their happiness. Given how much JK Rowling modeled her world on Tolkien's, it's no stretch to make the comparison with the Nazgul. And why do the Nazgul do this? I think that just as Gollum loves and hates the Ring, the Nazgul love and hate their deathless state and their Dark Lord, and that makes them envy the living with so much hatred that like the dementors, they suck out any hope and joy that the living might feel, even though they can't benefit from that hope themselves. It's more like spite taken to extreme: "if we can't have it, neither can you." And in Middle-earth, you can't live without hope. In re-reading this chapter, I was relearned that the Black Breath isn't just a bad case of melancholy but a fatal condition:

Quote

"They themselves do not see the world of light as we do, but our shapes cast shadows in their minds, which only the noon sun destroys; and in the dark they perceive many signs and forms that are hidden from us: then they are most to be feared. And at all times they smell the blood of living things, desiring and hating it."

What really drives LOTR or TH forward of course is not fantastical powers like those of Superheroes: it's qualities present in the real world, like loyalty, courage, determination and good moral compasses. And that enables Tolkien to have the hard work done by brave but 'normal' people.

the presence of hope in the middle of discouragement can almost have a weakening effect. You're not really sure if your efforts will succeed or which efforts are best to pursue. He didn't have the relentless tunnel-vision focus of someone like Eowyn who only wants to honorably die in battle.