Norovirus, also known as the stomach bug, is spreading across the U.S. Here's what to know about the contagious virus and how best to prevent an infection.

"It is too early to tell if this strain is associated with more severe norovirus disease but likely lower population immunity is the reason for the early surge of an otherwise seasonal increase of this virus," Jan Vinjé, head of the CDC's CaliciNet system, told CBS News in an email.