Nurgle uses Poxfulcrum as a test subject for the plague he brew by force-feeding samples from his cauldron, so he can observe its effects without causing lasting damage. For this purpose he has bestowed upon this daemon two gifts: the power to be extremely suscepptible to plagues and the ability to overcome them. Usually it recovers to quickly, leaving the plague god frustrated, through sometimes the suffering and agonies the demon goes through will pique his interest and he will work on refining his brew.
In Warhammer Fantasy Battles Poxfulcrum was introduced as a She-Daemon and rumoured to be the daughter of Nurgle, an entity like the Newborn. The End Times revealing her identity as Shallya. In Warhammer 40k, Poxfulcrum's role is taken by Isha.