Some giant rats carry vile diseases that they spread with their bites. A diseased giant rat has a challenge rating of 1/8 (25 XP) and the following action instead of its normal bite attack.
Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or contract a disease. Until the disease is cured, the target can’t regain hit points except by magical means, and the target’s hit point maximum decreases by 3 (1d6) every 24 hours. If the target’s hit point maximum drops to 0 as a result of this disease, the target dies.