Cases of norovirus — the virulent illness that causes vomiting, diarrhea, and stomach pain — are surging in the U.S. ahead of the holiday season, and it can’t be killed by hand sanitizer alone. 

Levels of the virus are “high” in wastewater, according to Today, which says that in the coming weeks, cases are expected to rise. “It’s a devilish virus because it can spread in so many ways, and it is so highly contagious,” Dr. William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center, tells Today. 

It commonly spreads in close quarters. Outbreaks frequently happen in dorms and on cruise ships. An AIDA Cruises ship currently on a 133-day world tour with stops in the U.S., England, Mexico, Japan, South Africa is dealing with an ongoing outbreak of the virus that has sickened more than 100 passengers and crew members. 

Although it’s sometimes called the stomach flu, norovirus isn’t caused by the influenza virus.

“You can get norovirus by accidentally getting tiny particles of feces (poop) or vomit in your mouth from a person infected with norovirus,” the U.S. Centers for Disease Control says. “If you get norovirus illness, you can shed billions of norovirus particles that you can’t see without a microscope. It only takes a few norovirus particles to make you and other people sick.”

“It takes very few particles, between one and 10, to initiate an infection,” Schaffner says. That’s why it also spreads so easily via food, when meals are prepared by an infected person who didn’t correctly wash their hands. 

Unfortunately, it’s not so easy to avoid getting sick, as alcohol-based hand sanitizers aren’t effective against the bug, experts say: “You have to use soap and water, which literally picks up the virus and washes it down the drain,” Schaffner said. The CDC’s guidance reiterates this, saying, “Hand sanitizer does not work well against norovirus. You can use hand sanitizers in addition to hand washing, but hand sanitizer is not a substitute for handwashing, which is best.”

Other ways to reduce your chances of getting it, along with washing your hands after using the bathroom and before eating, preparing or handling food, are to wash fruits and vegetables. Seafood should be cooked thoroughly — and if someone in your home has norovirus, clean and disinfect surfaces and wash all laundry in hot water, the CDC says.

Although vile, a norovirus infection is “relatively brief,” Schaffner tells Today. “You’re miserable for about 48 hours, then you get better.”