Sewer Plague

Sewer Plague is a set of illnesses that incubate in sewers, refuse heaps, and in other rancid places. They are typically grouped into the same disease for they all have very similar effects. These diseases are transmitted through the bite of creatures that live in such squalor and through contact with the waste. This disease is specific to humanoids. Rats and other such creatures will not die from the illness, but can transmit it to humanoids.
A creature exposed to Sewer Plague must succeed on a DC 11 Constitution saving throw or contract the infection at stage 1. While infected, the creature must repeat the saving throw once every 24 hours. On a successful save, the creature's condition decreases a stage. If they fail the save, the condition progresses a stage. If the creature successfully saves while on stage 1, the disease is shaken off and they are no longer infected.

Type

Bacterial

Stage 1
The infection creature begins to suffer from a fever, fatigue, and cramps. These symptoms give the creature disadvantage on attack roles and ability checks.
Stage 2
As the disease begins to rot the infected creature from the inside out, it can only regain half the rolled amount from hit dice and does not heal on a long rest. 
Stage 3
The infected creature suffers from more and more fatigue and their fever rises to severe levels. At this stage, they gain a level of exhaustion every 24 hours.