Epidemius is Nurgle's chosen Tallyman, and one of the seven Proctors of Pestilence who preside over the Plaguebearers. He is charged with cataloguing the potency of Nurgle's many diseases, and is constantly moving through both Daemon and mortal worlds, seeking out new strands of virus, fresh species of bacteria, and innovative symptoms of contagion.
Epidemius rides on a rotten palanquin borne aloft by Nurglings. The Nurglings also serve as his guards, and act as his assistants by secreting ink for his quill, growing parchment-like strips of skin from their backs that could then be torn free, and counting on a death's head abacus on the back of the palanquin. Unlike the constant babbling and giggles of usual Nurglings, however, these are silent, as they understand the importance of Epidemius's task, and suffer his ire whenever they break his concentration.
Even as Epidemius surveys and takes notes of the spread of filth and decay, Nurgle becomes aware of his learnings, and distills the information for future experiments and brews. For this reason, Epidemius focuses on his task even in the midst of battle, for if he were to make an error or untimely observation, Nurgle's anger would be dire indeed.