Vomiting and Diarrhea occurs when very strong contractions in the stomach force a large part of the contents of the stomach, back up through the esophagus and out through the mouth or nose. Vomiting can be aggravated if your body loses a lot of fluid. When there is very little fluid in the body, dehydration occurs. Gastroenteritis is the main cause of diarrhea in children and adults, and it spreads throughout the world. The infection irritates the stomach and digestive system. People who suffer from vomiting due to gastroenteritis may also have diarrhea. Medications, and other substances such as alcohol, can also irritate the stomach and cause vomiting.