From my notebook.

I had a dream.
Soldiers were deployed - urban ruins, there were monsters, sort of - a disease which controlled its host, a vicious hunter without a body. The victims become the predators - the soldiers' company is trying to get to safety, but they've suffered losses, the most recent of whom is stalking them from the shadows like Gollum, waiting to get the upper hand, behaving like a hyena or some other cautious but feral scavenger/hunter, like a wild dog. The company's progress is slow because they have with them a partially infected soldier - he sleeps to keep the parasite dormant - it gives him superhuman abilities like other victims, but for some special reason, some unique circumstance owing to blind luck, the infection's control over his mind is weak(er). When he wakes up he can smell the other infected hosts and can hunt them with the strengths granted him by the parasite, but the more physically active he is, the more the parasite consumes his body in its ongoing bid to control it.
Other victims' bodies are attacked only until the brain's control is wrested. This special soldier's brain has remained in charge, perhaps only because the parasites have not yet had success with the brain, for whatever random fluke of biology and genetics.

So the squad's captain or whatever finally decides to wake up the sleeping guy because they've been cornered in this one room of a bombed-out building - formerly a stair-well from the looks of it.
The infected soldier has chosen this opportunity to take shots at the squad, company, whatever. I guess it's small, so that's like a squad, right? About 5 guys now (men and women, actually) 7 or eight originally (when they set out, before the dream takes place).
The soldier wakes up as little as he can to defend the team. He tries to snipe the infected with his sidearm, his pistol, without opening his eyes all the way or focusing his mind on the task.
When that proves impossible, he grudgingly wakes up more, and gets up off the stretcher they've been carrying him on. (His pistol ran out of bullets.) Still refusing to fully direct his mind to a focused task, he tries to keep his mind relaxed and as close to unconscious as possible while still carrying out his task. Dreamily, he leaps to the broken-off balcony of the second floor without the aid of the stairs [which were rubble anyway].
Quickly, and without thinking (mostly) he draws his bootknife and charges the startled infected soldier [who had backed around the corner of the stairwell door into the remnant of a second-floor hallway, which was about 20 ft. long and ended in a T-intersection which was broken off on both sides, letting sunlight in from the dim, gray sky], tackling her to the corner where the wall meets the floor. [at the intersection where the other hall would cross it if there were another hall. They were both half illuminated. The sky looked like it could rain at any moment.]
Her expression is feral, but startled - her eyes look distant - she has a blonde miitary crew-cut, I think.
The special soldier relaxes most of his body the instant he makes the tackle, both to get ready to fall back asleep more easily, as well as to use his dead weight to incapacitate the infected one. Not that that's necessary - as soon as they hit the floor, he pressed his knife blade into her throat.
Her wild, frightened look relaxes into a peaceful, present look, as the parasites flee her dying body, releasing her from their control. She smiles and closes her eyes as blood begins to flow out her mouth and neck.

The was the end of the dream [this is still from what I wrote when I woke up].
A note; for the special soldier, trying to hunt and kill without waking up felt similar to the feeling of trying to record a dream, that same attempt to loosely organize thoughts and actions without shifting your awareness completely over to the waking world, or else you'll be unable to summon the sleep-thoughts which are vital to the process.

Other notes: Parasites could be alien - no inherent mechanism for seeking the brain. Wouldn't have to be alien for that though. Sort of like sperm bombarding until one makes it - the parasites swarm destruction through the body until some find the control center, at which point it sends signals throughout the appropriated channels which are conveyed to the other attacking parasites, calming them and eventually converting them for other uses to the ... colony. Such as breeding and ability enhancement (pain dulling, allowing greater feats of strength and endurance, to the dentriment of the host's muscle fibers, which the parasites attempt to patch up in a makeshift sort of way - possibly using themselves as replacements for destroyed fibers, also enhancing the receptivity, signal strength and nervous response time of the sensory organs - etc.)
It can be contracted from close proximity to a dying infected host though this is a lower risk, since most of the fleeing parasitic microbodies will already have been converted from Stage 1 invaders by the control signal into their various more specialized roles. Only the controllers are a real threat (though the others may attach themselves to a nearby viable host, if the genetic conditions are similar enough, without the controllers they will do nothing), and even then only if genetic factors are similar enough for the now-specialized microbodies to find something they can keep a hold on.
The real threat of infection comes from being near an infected host during Stage 3, when the specialized breeding mircobodies are manufacutring millions of non-specialized Stage 1 invasion microbodies.
During Stage 3 a host is a cloud of infectious death - this cloud could make them unreasonably difficult to combat - if I wanted to scale it back I could say that the approproation of so many of the body's resources during Stage 3 makes the host body sluggish, lethargic, or even catatonic. No such detail was provided by my dream, but that would make things a little more manageable.
Stage 3 could be cyclical (my first inclination) or it could mark the end of the parasites' destructive lifespan. No detail was given about this either, though I think cyclical makes the most sense. It feels more right. The bit about immobility during the high-contagion phase seems purely down to my preference. Maybe immobile, followed by a surprising change toward the ambulatory.
I don't know why they would be violent or semi-intelligent though, that's just how the dream had it.