Shawn's off fishing with Henry - or at least he's supposed to be fishing, but there's apparently an Olympic women's volleyball training camp nearby. Anyway, Gus is stuck in Santa Barbara, delivering pharmaceuticals as more and more of the town (and beyond) are coming down with a cold. Eventually, it becomes obvious that this flu isn't a minor inconvenience, and people begin to die. And soon, it's more of an apocalypse than Gus would have imagined.
Warning: Character death. My favorite character in Psych :( But the story's still good, and it manages to have a happy-ish ending.
Pocketful of Posies...and Plague, by wizefics