Backstory...
At some point the Cult found itself becoming the infested, rather than the infesters, when their bulk lander was swallowed by a Warp storm that stranded them on the outskirts of Nurgle’s Garden.
The cult discovered the true meaning of parasitism and horror.
Eventually, the Grandfather of Plagues allowed them to emerge into realspace once more, horrifically changed and ready to serve their new master’s sickly agendas