Monks of the Way of the Rat specialize in desperate and viscous combat. They have honed their ki to mask them and to induce fear in their enemies and, with enough training, even carry disease. Monks of the Rat are not fair fighters, running when outmatched and striking at any opportunity. Like rats, they have no problem groveling or debasing themselves for the chance of a cheap hit. 

Verminkind

Starting when you choose this tradition at 3rd level, your training makes you more rat-like, giving you darkvision out to a range of 60 feet as long as you can see. Additionally, you have advantage of perception checks that rely on smell.

You may also replace one of your unarmed attacks in your Flurry of Blows with a bite attack dealing 1d4+[str]. This can be done once for each time you use your Flurry of Blows.

Pest Control

Also at 3rd level, you gain an understanding of rats and they you. You may expend 1 ki point to attune yourself with a rat that you can see to mimic the effects of the Speak with Animals spell. You may expend additional ki points to speak to additional rats.

At 9th level, You may expend 5 ki points to summon 1d4 swarms of rats. The swarms are friendly to you and your allies and attack on your initiative count. A swarm may be dismissed as a bonus action. If this ability is used out of combat, the swarm can perform simple tasks, the limits of which are at the DM's discretion.

Rat in the Shadows

At 6th level, when you are in dim light, you may take the hide action as a bonus action and if a creature would make an opportunity attack against you, they do so with disadvantage while you are in dim light or darkness. Additionally, you can fit through spaces meant for creatures of one size smaller while in dim light or darkness. 

Musophobia

Beginning at 11th level, when you take the bite action as part of your Flurry of Blows and hit, you may spend 1 ki point to force the target to make a wisdom saving throw (DC equals 8+wisodom+proficiency). On a failure, they are Frightened and must use their action getting as far away from you as possible. This lasts for 30 seconds, after which they may make another saving throw, ending the effect on themselves on a success. On a success, they are immune to your Musophobia for 24 hours.

Plaguebringer

At 17th level, if you spend at least 1 minute rooting around in the sewer or similar filth, you gain the ability to inflict the Poisoned condition if you take the bite attack when you use your Flurry of Blows. Additionally, you may spend 5 ki points to inflict the Sewer Plague on a target of your choosing through touch.