From Lexicanum

Nurgle's Rot is the most foul of the countless contagions which afflict mortal beings. It is Nurgle's gift to the mortal universe. It is completely incurable, highly infectious, has a very slow course that turns the infected into bloated, rotting, corpses,^1a and worst of all it corrupts the soul of the infected into new Plaguebearers. ^1b

It is as much a spiritual plague as a physical one, as the souls of those infected are slowly leeched into Nurgle's realm, where they appear as warty seed pods growing from cracked branches of gloomy willows. Each pod swells and ripens as the plague destroys its host in the real world and the nascent Plaguebearer feeds upon the victim's dying energies. When fully mature, the podule drops and the newly created Plaguebearer tears himself free. A mortal who resists for a significant period produces an equally long incubation period resulting in a larger, tougher and more disgusting Herald of Nurgle.^1b

Sources

• ¹: Codex: Chaos Daemons (4th Edition)

^a:p.38, ^b:p.34

• Realm of Chaos: The Lost and the Damned