Gods' Rot

God's Rot was a nickname for an unnatural foul concoction of plague and pestilence that literally rots the a poor victim from the inside out. The first symptoms of the plague were similar to other types of common diseases; nausea, pain in the joints and belly, and so forth. At first it caused no alarm; however that was of course before the maggots set in. Unbeknownst the victim, each of their organs had already started its slow inevitable decay, becoming a breeding ground for the worm-invaders. If the victim is lucky, their heart was one of their first organs to rot, allowing them a relatively quick death. If not, then the rot starts in the entrails and the victim endures great pain while it slowly makes its way to the chest, causing may organs to fail in the process.

The disease known as, "Gods' Rot," was intentionally spread by Egrimm van Horstmann, chosen of Tzeentch and traitor of the Light College, to infect the many districts of Altdorf. Van Horstmann has daemons from each of the Chaos God's bound to him, and great pleasure was felt to such a bound plague daemon when van Horstmann gave it the stolen Skaven-tome, The Liber Pestilentius, to brew a particularly devastating disease. However, van Horstmann is no servant of Nurgle, and his goal was was never to infect all of the city, but rather infect certain districts that could easily be quarantined, but would maximize the fear and pandemonium that would be felt around the whole city. "Gods' Rot," was only a small piece of a much larger and masterly woven plot more fitting of a servant of Tzeentch...

Source

 * Van Horstman (Novel) by Ben Counter
 * Chapter Twelve