Originally from: http://en.wikipedia.org/wiki/Death_Guard

Mortarion
When the twenty Primarchs of the Space Marine Legions were scattered across the galaxy in a mysterious accident, one came to rest on the planet Barbarus, a world wreathed in poisonous fog. The population of the world was split into two groups: the controlling warlords; necromancers with fantastic powers along with the human settlers, who had been trapped on the planet millennia before and were now forced to eke out an existence in the poison-free valleys of the planet, fearing the wrath of the warlords and their creations. (more…)

Although it is not widely known, Mortarion himself has a host of diseases that are uniquely his creation. These maladies are far more rare than most diseases, as only certain followers of the Primarch are granted the ability (and indeed permission) to transmit and spread them. Although there are far more than listed below, these are a small sample of the most horrific of Mortarion’s plagues. (more…)

Surrounded by the thick smell of decay, the 4th Cohort crept silently through the endless marsh, a constant buzzing of flies surrounding their advance. Thick billowing fog had sat in over the wetlands, serving to obscure their movements. Initial scouting reports indicated a small force of Astartes, Silver Skulls chapter, within the area. Under the command of Chearon Morbidius, they sat low in the dank river, their rebreather equipped helmets barely visible in the dark water. (more…)