Anytime ten or more wererats are found together, one of them will be a 5 HD specimen with 26 HP with 1d4-1 royals, 3d6 chroma, 1d4+6 royals, and 3d4 copper (35% it's Red Cuprous, 20% it's Dark Cuprous). This leader will have the same Morale and saving throws as its fellows, but is BAB +5 and it's bite deals 1d2+4. A defeated leader wererat is worth 250 xp.
A wererat (plague or otherwise) is indistinguishable from humans by normal senses. A wererat's alignment detects as evil. In lieu of other actions in combat, a wererat can change forms. It may take a large rat form or a humanoid/rat hybrid form. If already in one of those two forms, it may polymorph into the other, or resume it's human appearance.
When bitten by a wererat, a PC must make a DC 12 Constitution based saving throw or will else contract lycanthropy.
Wererats have immunity to most mundane attacks (piercing, slashing, blunt). They are vulnerable to silvered and/or magical weapons. Additionally, damage dealt by a paladin or Lawful cleric has full effect, regardless of the quality of the PC's weapon. Fire does half normal damage if the wererat fails it's saving throw, one-quarter otherwise.