An outbreak of a nasty stomach bug among a girls' soccer team offers a good lesson in why it's never a good idea to store anything in a washroom that might later come into contact with food.

In a report detailed in the Journal of Infectious Diseases, Oregon public health investigators explain how they traced an outbreak of norovirus to a reusable grocery bag that was stored in a hotel room bathroom.

The investigators were trying to figure out how seven members of a team of 13- and 14-year-old soccer players became ill while on a trip to Seattle for a weekend tournament in October 2010.

They knew the outbreak started with a single infected player who had picked up norovirus, a stomach bug that can bring on gastroenteritis (otherwise known as "stomach flu," even though it is not related to influenza in any way). Symptoms of gastroenteritis typically include intense stomach cramps, vomiting and diarrhea.

The infected girl spent much of Saturday night vomiting in the bathroom of the team's chaperone, who decided in the morning to take the girl back to Oregon. (The chaperone later became sick herself.)

By Tuesday, six more players on the team came down with a stomach bug too.

What baffled the Oregon public health officials was why it took two full days for the other players to get sick. The first infected girl and the chaperone never rejoined the group or took part in any meals with them, so investigators wondered how the virus could have spread.

They narrowed it down to a package of cookies.

"On reinterview, we learned that the cookies, along with packaged chips and fresh grapes, had been stored in a reusable open-top grocery bag made from laminated woven polypropylene" the investigators write in their report.

Hours after the first girl had left -- the bag was taken to another hotel room where the cookies, chips, and grapes were passed around as part of the lunch. All six of those who later got ill helped themselves to some of the snacks.

Investigators learned that the bag had been stored in the hotel room of the chaperone, who had cared for the first sick girl. Even though the girl never actually touched or handled the shopping bag, it was in the bathroom she used the night she was vomiting and became contaminated.

Tests on the bag turned up the virus on the sides of the bag.

Norovirus is notoriously contagious. It leads to thousands of illnesses a year in Canada and is often the cause of outbreaks on cruise ships, day cares and nursing homes

The virus can be transmitted by direct human contact, but can also survive for hours and sometimes days on practically any surface, including door handles, sinks, and glassware.

"Aerosolization of vomit and feces has been demonstrated to be of major importance in norovirus outbreaks," the authors of this report write.

That's why it's important to fully disinfect areas where "aerosol exposures" may have occurred, they write -- not just exposed surfaces but any objects lying nearby as well.