Cases of the illness known as norovirus — which induces miserable bouts of vomiting and diarrhea — are surging across the United States, according to the Centers for Disease Control and Prevention. Ninety-one outbreaks of the gastrointestinal bug were reported the week of Dec. 5, the latest period for which data is available. That’s 22 more outbreaks than in the last week of November. While sometimes referred to as the stomach flu, the disease is not caused by the influenza virus, which results in respiratory illness.