Norovirus is associated in the public mind with cruise ships limping into port full of passengers who are vomiting or doubled over with painful stomach cramps. But while the gastrointestinal virus affects 20 million U.S. residents every year, only about 1 percent of them are on cruise ships, the Centers for Disease for Control and Prevention reported Tuesday.

Norovirus lives in the human intestine and is spread “primarily via the fecal-oral route,” according to the CDC. That includes direct person-to-person contact, consumption of contaminated food and water, and contact with contaminated surfaces. It also can spread by accidental ingestion of aerosolized vomit droplets, the agency said.

About 5.5 million of the annual cases are spread via food, said Aron Hall, an epidemiologist with the CDC’s viral gastroenteritis team.

CDC Director Tom Frieden called on the food service industry to redouble efforts to ensure that food workers wash their hands frequently and handle food with utensils or use gloves.

The CDC report said that “food workers continue to be the primary source of contamination and have the potential to significantly amplify community transmission of noroviruses through widespread exposure.”