What is it?
Easily spread virus that is an extremely common cause of food-borne illness. Affects gastrointestinal system in humans.

Symptoms
Acute diarrhea, abdominal cramps, nausea and vomiting, headache and low-grade fever. Symptoms usually show up 10 to 60 hours after exposure and last up to two days, but can persist for several weeks.

Sources
Person-to-person contact. Contaminated shellfish, water and ice.

How it spreads
In many cases, infected food handlers can contaminate food. According to the Centers for Disease Control and Prevention, the only source of the Norwalk virus is feces from infected people. In many cases, shellfish are infected when human waste is dumped from ships.

You can pass on the virus up to 48 hours after your symptoms disappear.

There are an estimated 181,000 cases of the Norwalk virus in the United States every year.

Treatment
The sports drink solution: there is no effective treatment, other than fluid and electrolyte replacement.

Prevention
Doctors say hand washing and sanitizing is the best method for preventing the spread of the virus.

Outbreaks
Outbreaks of the Norwalk virus usually occur where a large number of people are confined to a relatively small space. Outbreaks can occur anywhere contaminated drinking water or swimming water exist, and are known to affect camps, cruise ships, schools and nursing homes.

Recent outbreaks

Oct. 18, 2006: Health officials suspect that the Norwalk virus is to blame after at least 30 students fall ill at St. Francis Xavier University in Antigonish, N.S. Most of the students live on campus in university residences.

Oct. 13, 2006: More than 300 students at Mount Allison University in Sackville, N.B., are stricken with Norwalk virus. The school closed public buildings and cancelled extracurricular activities for several days. Extra cleaning staff were brought in and hand sanitizers were passed out to students to help combat the virus.

Sept. 26, 2006: More than 30 people come down with Norwalk virus at Victoria Hospital in Prince Albert, Sask. The problem started in the palliative care ward and later spread to the surgical ward. The hospital cancelled some surgeries that required overnight stays. It was the second outbreak of the virus in Saskatchewan in a month. A week earlier, 26 people fell ill at a health-care facility in Yorkton.

March 20, 2006: Norwalk virus affects about 90 staff and patients at two hospitals in British Columbia. Health officials close the contaminated wards to prevent the further spread of the virus.

Nov. 25, 2004: Up to 100 students and five teachers at a public school in Hamilton, Ont., become sick with Norwalk virus.

Nov. 9, 2004: Health officials confirm that Norwalk virus is responsible for illnesses at a homeless shelter in Calgary.

June 11, 2004: Ten patients at St. Paul's Hospital in Vancouver become infected with a Norwalk-like virus. Three staff members are sent home sick.

May 31, 2004: About 180 people become sick with Norwalk virus after visiting Emerald Lake Lodge in Alberta's Yoho National Park.

Oct. 11, 2003: More than 60 people at an RCMP training facility in Regina are treated for symptoms of a Norwalk-like virus. Most of the patients are RCMP cadets.

Dec. 16, 2002: A Norwalk-like virus strikes more than 200 passengers on the cruise ship Carnival Conquest.

Dec. 4, 2002: At least 60 passengers fall ill with the virus aboard the Disney cruise ship Magic in Florida. This comes just days after the ship was thoroughly cleaned when 275 passengers fell ill in a similar outbreak.

Nov. 28, 2002: Emergency room at Toronto's Sunnybrook Hospital is closed as officials try to contain an outbreak of the Norwalk virus.

November 2002: Holland America sanitizes the Amsterdam after more than 500 people catch the virus on four separate voyages.

July 2002: Holland America pulls the Ryndham from service to be sanitized after 388 passengers fall ill during an Alaska cruise.