"Nurgle is the Chaos god of disease, decay, despair, destruction and rotting death. He is the oldest of the four Chaos gods and is the most involved with the plights of mortals. Many of those affected by his poxes usually turn to him in order to escape the pain caused by sickness"
The imperial guards are infected by the poxes of Nurgle. They start to turn to the chaos and become extremely aggressive and lose most basic thoughts. They mainly wander around and attack anything on site including each other.