Dysentery is one of the Afflictions that can afflict the player in The Long Dark. It is caused by drinking Unsafe Water. While the player will not always get dysentery from this, it is extremely likely to occur.
To treat the condition, the player needs 4L/1.053gal of Potable Water and either two Antibiotics or a cup of Reishi Tea. Once the medicine is taken, the player will stop losing Condition, and may recover it naturally. However, Thirst will still decrease until the player rests for a period of time, usually ten hours. The Potable Water's affect on thirst still occurs (as well as granting the player calories for Hunger if the Reishi Tea was chosen), so the player will max out their thirst meter, although it will recover far less than the player would have normally achieved for that amount of water.
Because the player will not lose Condition after taking the medicine, it is advised to rest in bursts, so that the player can replenish lost water if they need to.