Giant rats are scavengers, but will attack to defend their nests and territories. A giant rat can grow to be up to 4 feet long and weigh over 50 pounds. A single giant rat, or a small group of up to four, will generally be shy, but larger packs attack fearlessly, biting and chewing with their sharp incisors.

Any rat bite has a 5% chance of causing a disease. A character who suffers one or more rat bites where the die roll indicates disease will sicken in 3d6 hours. The infected character will lose one point of Constitution per hour; after losing each point, the character is allowed a save vs. Death Ray (adjusted by the current Constitution bonus or penalty) to break the fever and end the disease. Any character reduced to zero Constitution is dead. See Constitution Point Losses in the Encounter section for details on regaining lost Constitution.