Close up image of a caretaker helping older woman walk

Stomach viruses have infected a wave of people across the country in recent weeks, including many residents and workers in nursing homes.

Twenty nursing homes in California’s Contra Costa County, for example, have reported outbreaks, twice as many as in a normal year. In Montana, cases began in nursing homes the week before Christmas. Six facilities have confirmed positive cases.

Noroviruses are the most common cause of contagious gastroenteritis, also known as the stomach flu. Cases hit every winter, but there have been two to three times more cases than usual, health officials say. Outbreaks in nursing homes are of particular concern because of the risk of dehydration caused by vomiting and diarrhea, according to health experts.