Also called Viral Gastroenteritis, many argue the stomach flu is one of the worst minor illnesses one can get. This gastric flu or stomach virus has many symptoms including, abdominal pain, nausea, cramping, diarrhea, chills, low-grade fever, headache, muscle aches and weakness.  How does this begin? Well, gastroenteritis is inflammation of the stomach and intestines, caused by viruses.