Scott is posting twelve posters, each on a different blog, and each foreshadowing a key plot element. If you get all twelve and put them together in the right pattern, you get a final clue for the book's big finish. this list of other blogs hosting a poster.
Here's the promo stuff for the book: "Contagious is a hard-science horror novel with a popcorn-flick flair. From the book jacket: Across America, a mysterious pathogen transforms ordinary people into raging killers, psychopaths driven by a terrifying, alien agenda. The human race fights back, yet after every battle the disease responds, adapts, using sophisticated strategies and brilliant ruses to fool its pursuers. The only possible explanation: the epidemic is driven not by evolution but by some malevolent intelligence. "