Pestilence

As war grinds on in the Genovingian sector, Imperial medicae Lemual Sark investigates Uhlren's Pox, a mysterious and deadly plague that is striking down the Imperial Guardsman who fight the war. As he narrows down the source of the contagion, Sark finds himself thrown into a plot by to wipe out the defenders of the sector and allow the Ruinous Powers their triumph.

Read it because
It's an intriguing look behind the lines at how the forces of Chaos fight in many different ways, and salvation sometimes comes from sources you don't expect.