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

Much more common is person-to-person and food-borne transmission of the virus, the latter primarily because of poor hygiene among food workers who touch foods with bare hands right before you eat it. About 5.5 million of the annual cases are spread via food, said Aron Hall, an epidemiologist with the CDC's viral gastroenteritis team.

In a telephone news conference, CDC Director Tom Frieden called on the food service industry to redouble efforts to ensure that food workers wash their hands frequently, handle food with utensils or use gloves when they touch your food.

"There's nothing worse than going out to eat and coming back with, instead of a good meal, an infection that's going to affect you for a few days," Frieden said.

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

The CDC looked at 4,318 reported norovirus outbreaks from 2009 to 2012, which resulted in more than 161,000 illnesses, 2,512 hospitalizations and 304 deaths, according to the report. (Children under age 5 and adults older than 65 are most likely to be killed or admitted to hospitals, the report showed.) Food-borne transmission was responsible for 23 percent of the outbreaks, more than half of which occurred between December and February of each year.

"Food workers continue to be the primary source of contamination and have the potential to significantly amplify community transmission of noroviruses through widespread exposure," the CDC reported concluded. Most of the food-borne spread of the pathogen occurs in restaurants and via catering, where food workers touch food such as salad vegetables, fruit and food that already has been cooked, immediately before it is consumed, the report said.

A common problem noted by Frieden is that workers who have suffered from the virus themselves return to work less than 48 hours after their own vomiting and diarrhea have stopped because they fear losing their jobs or don't want to leave co-workers short-staffed. He urged food companies to take steps to prevent that by enforcing the 48-hour standard and having fill-in staff on call.

Hall said CDC recommends washing raw foods, such as salad vegetables, right before you eat them, even if they are packaged and advertised as pre-washed.