Gastroenteritis (also known as "stomach flu") is a medical condition characterized by inflammation of the stomach and the intestines. It is caused by drinking contaminated water.
While similar in its course, the gastroenteritis is way less severe than cholera. It will go away on its own after some time passes, if the player manages it correctly. However, drinking more contaminated water may lead to cholera occurring. Having this disease doesn't remove the possibility of the player obtaining cholera.
As the main danger caused by the disease is dehydration, the player must make sure that his character has plenty of clean water to drink.
Taking of antibiotics pills or drinking tannin tea will boost the character's Immune System allowing his body to recover from the disease faster. Resting in a campsites with higher shelter value will have similar effect.
It is also good to remember that vomiting and diarrhea caused by the gastroenteritis, make the character much easier to track.