All rats have fleas, but the fleas that inhabit Morbus's matted fur are monstrous mutants among their kind, having feasted on the warpstone-corrupted blood of the Plague Priest. They even affect other Skaven, because the fleas have grown so large from the effects of the warpstone in Morbus' bloodstream.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
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All models in base contact with Morbus suffer a -1 To Hit penalty in close combat as they are distracted by the biting fleas.