“Commonly known as the stomach flu, norovirus has been around forever, and has become the leading cause of gastroenteritis or inflammation of the intestinal tract that causes vomiting and diarrhea,” says Dr. Edmund Milder, a pediatric infectious disease specialist at Rady Children’s Hospital and UC San Diego Health.

The highly contagious stomach bug calls for “enhanced precautions” — the highest level of cleaning.

Despite millions of cases of norovirus in the U.S. each year—and a current surge in outbreaks—there is still no vaccine for the stomach bug. Here's why it's taking so long, and how close we are to a norovirus vaccine.