Derek banished his betas the moment he recognized his symptoms. The wolf flu is highly contagious and a grinding misery while it lasts. He's had it four times before, but never since his family died; it's a hazard of living among other werewolves. He's willing to accept the tradeoff, on some philosophical level, but for now he's alone, shivering and trying to sleep when he can feel every obnoxious irregularity of the floor he's lying on, can smell every living thing within a hundred yards all at once, can hear his own heartbeat pounding so loudly it seems to echo.
It's not echoing, he realizes. He's hearing another heartbeat coming closer. Not his pack--he was very convincing when he warned them off--or at least, not a werewolf.
"Stiles," he croaks, as Stiles's footsteps pound down on the bare floor like thunderclaps.
"Yep, that's me," Stiles agrees, and when he speaks Derek can't hear anything else. "One hundred percent human and one hundred percent immune to werewolf sickness, or so Dr. Deaton promises me. I brought you a blanket."
"You what," Derek says, slitting his eyes open even though the light is a vicious stab, because he has to see this. Sure enough, Stiles is standing there holding a blanket folded against his chest. He's got a thermos, too, but it's so tightly sealed that Derek can't smell what's in it, can't even feel heat bleeding off it. He feels the heat radiating from Stiles, though, pleasant as a woodstove in winter.
Derek shivers, and Stiles comes forward immediately, dropping the thermos with a horrible clatter. He unfolds the blanket, flapping it out over Derek like a banner and letting it settle slowly, covering him from ears to toes. It carries a little of Stiles's body heat, but mostly Derek is engulfed in the smell of it, conscious of nothing else for a moment.
The top layer is the smell of Stiles himself, which overlays everything right now, and which Derek automatically filters out after a few heady seconds. The smell under that is the Stilinski house--compounded of familiar bodies and cleaning products and favored foods and the exhalations of the house itself, wood and plaster and paint and tile all giving up their particular chemical compounds and adding up to a house that has been inviolate for all the time since it was built, a house that has always been a home, a sanctuary, a refuge.
Derek curls up tighter, pressing his face into the blanket, only vaguely conscious of Stiles tucking it around him. He falls further into the specific smells, the old, faint individual ones. As hyperaware as he is now, every smell is a picture in his mind, the faintest whiff blooming into an entire scene. There's a human scent he knows: Sheriff Stilinski, touched with weariness and a breath of whiskey. Stiles settled this blanket over him, too, one night when he'd stayed awake too late, working or just worrying. Stiles tucked the blanket around him, took the bottle away, slipped his shoes off and tucked the blanket around his feet before creeping quietly away.
Derek turns his head and finds another smell, older, less lonely but more openly grieving. It smells like Scott, though entirely human and younger than Derek ever knew him--Scott and Stiles and grass stains and tears, freshly-washed formal clothes and a distant hint of dry dirt. The boys lay together on this blanket in the backyard, staring up at the stars the night after Stiles's mother's funeral, curled close as puppies while Stiles cried and Scott kept quiet vigil, not trying to console Stiles except with his steady presence.
Stiles smooths a hand over Derek's shoulder, and Derek turns his head and finds another scent on the blanket--another fever, this one Stiles's, and another gentle hand. Stiles was home sick from school, curled up on the couch and feeling miserable, and his mother tucked this blanket around him, soothing him with a touch the way only a mother could. Stiles accepted the affection as his due, and had no idea that he should be cherishing that moment, no shadow of fear of what might happen. He was a child, and his mother could do anything for him and always would.
The blanket flaps up and cold air attacks the small of Derek's back. He winces away and gasps, and then Stiles is pressing up against his back, warm and solid and familiar. He tucks an arm around Derek as the blanket settles over them both.
"I know it's, like, five years too early to be saying anything about sickness and health," Stiles murmurs against the back of Derek's neck. "But I'm staying with you anyway, okay?"
Derek nods, because he can't smell anything on the blanket now but them, sinking into it, weaving into it with every other part of Stiles's family. Neither of them needs to say a word; the truth of this is all around them.