Around 998.M41 elements of the Genestealer Cult Tenebrous found themselves becoming the infested, rather than the infesters, when their bulk lander was swallowed by a Warp Storm that stranded them on the outskirts of the Garden of Nurgle in the Realm of Chaos.
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.
- Codex: Genestealer Cults (8th Edition), pg. 37, "Infestation and Plague"