Cases of norovirus have surged in the first week of December alone, according to the U.S. Centers for Disease Control and Prevention.

Most norovirus outbreaks occur when people who are already infected spread the virus to others by direct means, such as through sharing food or eating utensils.

The number of national norovirus outbreaks was the highest seen in December since at least 2012. Experts suggest increased hand-washing to avoid catching the ultra-contagious stomach bug.

As people plan to get together to ring in the new year, the CDC is warning of a surge in norovirus cases across the country.