Dean starts coughing while they're at the county records office, xeroxing the local sewer system maps. Just the once to begin with, short and hard-edged, and it takes them both by surprise. But Dean doesn't make anything of it and so Sam turns back to his task of trying to convince the copier to stop cutting the edges off the detailed map of a particularly convoluted section of sewer. By the time they leave an hour later, though, Dean's coughing with some regularity, in tight little bursts that he muffles against his elbow.
Not a fic I likely would've come up with on my own, but I'm stilll quite pleased with how it came out. It was an interesting task to write brotherly affection without sliding into schmoop. And there was something very satisfying about bucking the usual tropes and having the guys react in a practical and functional-adult way to a minor illness.