From the preview:
The Banebrood are immune to almost every disease in existence.
As the centuries passed some of them began to revel in pestilence, and it was not long before some enterprising misanthropes began to use disease offensively, by deliberately infecting mortal cities.
It was the Ramtaurs who became the masters of this new weapon, and mortals name them Pestbringers.
On the battlefield they intentionally infect friendly troops with diseases which are then spread to enemy troops. Quite in addition they can hurl parts of dead corpses at the enemy, splattering them with virulent plagues.
Pestbringers are no walkovers in melee with their 17 life-points and decent fighting skills in addition to a stench which forces enemies to pass a leadership test in order to attack them.