PORTLAND, Ore. (AP) -- Oregon investigators have traced an outbreak of norovirus to a reusable grocery bag that members of a Beaverton girls' soccer team passed around when they shared cookies.

The soccer team of 13- and 14-year-olds traveled to Seattle for a weekend tournament in October 2010.

At the tournament, one girl got sick on Saturday and spent six hours in a chaperone's bathroom. Symptoms of the bug, often called "stomach flu," include vomiting, diarrhea and stomach cramps. The chaperone took the girl back to Oregon.

On Sunday, team members had lunch in a hotel room, passing around the bag and eating cookies it held. On Monday, six girls got sick.

Oregon scientists determined they had picked up the norovirus from the grocery bag.

Tests turned up the virus on the sides of the bag below the polypropylene handle.

The results of the research have been published in the Journal of Infectious Diseases.

The authors are Kimberly Repp, epidemiologist for Washington County, and William Keene, senior epidemiologist with Oregon Public Health.

Norovirus causes about 21 million illnesses, 70,000 hospitalizations and 800 deaths a year in the United States. It caused 139 of 213 outbreaks of gastroenteritis in Oregon in 2010.

The germ can spread quickly in places like day care centers, nursing homes, and cruise ships.

Usually, it's transmitted by direct human contact, but can contaminate surfaces. Leafy greens, fresh fruits and shellfish are commonly involved in foodborne outbreaks.

An epidemiologist at the Centers for Disease Control and Prevention said the work also shows how hardy the norovirus is and how difficult it is to control.

"What this report does is it helps raise awareness of the complex and indirect way that norovirus can spread," said Aron Hall, an epidemiologist with the Division of Viral Diseases at the Centers for Disease Control and Prevention.

His agency says the best way to fend off the virus is thorough hand-washing and cleaning contaminated surfaces with a bleach-based solution.