If your Army Faction is DEATH GUARD, while an enemy unit is within Contagion Range of one or more DEATH GUARD models from your army, it is Afflicted (see below).
Contagion Range changes over the course of the battle:
In the first battle round, the Contagion Range is three inches.
In the second battle round, the Contagion Range is six inches.
From the third battle round, the Contagion Range is nine inches.
During the Declare Battle Formations step, select one of the Plagues below. Until the end of the battle, while an enemy unit is Afflicted, subtract 1 from the Toughness characteristic of models in that unit, and that unit has the effect of your chosen Plague.
This horrifying affliction sees the minds and senses of the victim gnawed gradually away from within.
Each time a model in this unit makes an attack, subtract 1 from the Hit roll.
Limbs shuddering with fever palsy, bones turned brittle as glass, these warriors can barely act to defend themselves.
Worsen the Save characteristic of models in this unit by 1.
Victims of this insidious ailment become glassy eyed and morose as their very animus and essence decay.
Worsen the Move, Leadership, and Objective Control characteristics of models in this unit by 1 (this rule can only worsen a model’s Objective Control characteristic to a minimum of 1).
When mustering your army, unless specifically stated otherwise, you cannot select PLAGUE LEGIONS as your Army Faction.