At least 30 stomach illnesses have been traced to an outbreak of norovirus that occurred last weekend at a Woodbury Chuck E. Cheese's theme restaurant.

Minnesota health officials learned on Monday of a suspected norovirus infection in a person who had been at the restaurant, and passed the tip along to the Washington County Public Health Department.

Working with the restaurant to gain contact information via reservations, county workers reached out to 60 households and found about half of them contained someone with gastrointestinal illness.

The restaurant closed Tuesday evening and was cleaned thoroughly before reopening Wednesday afternoon.

But even if the source of the outbreak has been eliminated, health officials said it is still important to find out how many people were infected and how the virus might have spread so broadly.

"Part of the investigation is 'How can we be doing things better?' " county health spokeswoman Jean Streetar said. "What was overlooked, potentially, at the restaurant? We want to correct these things and learn from them as we go forward."

The outbreak appeared to occur largely last Saturday, but health officials are reaching out to those who were at the restaurant on other days, just to be sure. None of the suspected norovirus cases so far resulted in hospitalizations or severe complications. Streetar didn't immediately know how many of the sickened people were children.

Noroviruses are common causes of food poisoning and spread rapidly through contact with contaminated surfaces or the consumption of contaminated food or beverages. The viruses are found in the stools or vomit of infected people, so a primary prevention method is thorough hand-washing before preparing or eating food.

Restaurants and large food gatherings are common sources of norovirus outbreaks, which also have bedeviled cruise ships. Health officials in Hennepin County investigated a similar norovirus outbreak at a Chuck E. Cheese's in Maple Grove last spring. State health data show that about half of the roughly 40 foodborne disease outbreaks each year are traced to norovirus.