Also, people often confuse being sick with a bad cold or stomach flu with having influenza. Influenza symptoms can include a fever, chills, sore throat, runny or stuffy nose, body aches, fatigue, and headaches. Cold symptoms can be similar to the flu but are typically milder. The stomach flu, or gastroenteritis, can be caused by several different bacteria or viruses. Symptoms of gastroenteritis involve nausea, vomiting, and diarrhea.