They are often seen feeding on the dead but will sometimes attack a very much alive target. They bear down on them with weight of numbers and any who cross paths with them are in dire threat due to this very fact.
Clan Pestilens have taken these rats and use them as vessels to spread contagions and lethal diseases. The Plague Priests infect them and then release them into the swarms of rats that already came with the Skaven. The plagues then spread and anyone bitten by these creatures will succumb to the fever carried to them.