Diarrhea is a medical condition where bacteria is ingested in infected water, causing irritation of the player's large intestine. Water passes into the feces from the intestine which causes quick dehydration. Diarrhea is often caused by cholera, an infection of the digestive system.
Diarrhea causes the player to vomit in the game, which increases thirst rate. So the player must find nearby water sources like lakes or marshes. The water may be contaminated, which could make the condition worse, so its recommended to sterilize it by boiling it with a Metal Sauce Pan/Soup can, testing it, or purifing it with a Water Purification tablet. As long the player stays hydrated, the diarrhea goes away on its own as long the player does not have the cholera condition, since it accompanies diarrhea.