CHICAGO – Parents, beware: Kids are back in school and stomach flu cases are on the rise. Rates of norovirus infection – the most common cause of the stomach flu – have been noticeably higher this winter than in previous years, said Dr. Alfredo Mena Lora, Saint Anthony Hospital’s director of infectious disease. The virus has surged across the United States, with 91 individual outbreaks recorded in the first week of December, the highest spike in cases since at least 2012, according to the CDC.