As a severe outbreak of norovirus, also known as the stomach flu, spreads across the United States, doctors have shared advice with Newsweek on what to look out for about the illness. The highly contagious illness has reached its highest levels for this time of year in more than a decade,

Norovirus outbreaks are surging in the United States this winter, with 91 cases reported in early December, a significant rise from previous years, according to CDC data.

Norovirus is the leading cause of foodborne illness in the United States, responsible for 58 percent of such infections acquired in the country each year, according to the CDC.