Sacramento County public health officials are investigating Mulvaney's B&L restaurant as the source of a possible norovirus outbreak in late February.

Norovirus is a highly contagious stomach virus that causes vomiting, diarrhea and nausea.

The investigation centers on diners who ate at the popular midtown restaurant on Feb. 23 and Feb. 25-26. The county Department of Health and Human Services received 20 reports of customers falling ill shortly after those days and launched the inquiry. Given that Mulvaney's has a 100-person seating capacity, hundreds of patrons may have been exposed.

"This is the nightmare that keeps you up at night," Patrick Mulvaney, the restaurant's chef and proprietor, said. "Where this started, we don't know."

According to Dr. Olivia Kasirye, county public health officer, two people connected to the restaurant have tested positive for norovirus. Mulvaney said one was an employee and the other a diner. The name and nature of the employee's work is confidential, and the final investigation results, including the possible number of infected people, are expected within two weeks.

Given that no further incidents have been reported at the restaurant at 1215 19th St. since late February, Mulvaney's B&L remains open. The restaurant passed its most recent county inspection on Feb. 13 and has continued to operate during the norovirus investigation.

"We don't usually shut (a restaurant) down unless we feel there's an ongoing risk to the public," Kasirye said. "We have not received any reports of illnesses since then."

According to the Centers for Disease Control and Prevention, norovirus sickens 21 million Americans annually and is responsible for 800 deaths. Pinpointing the source of an outbreak remains challenging. Norovirus can be transmitted through contaminated food or surfaces, along with personal contact. The virus can live up to three weeks in the body once symptoms have passed.

Nursing homes and day care centers are common places for norovirus outbreaks. Norovirus was suspected in sickening more than three dozen adults in February at two assisted-living facilities in Sacramento.

Outbreaks are fairly rare for restaurants. County public health officials conducted only one norovirus investigation at a local restaurant in 2012. That undisclosed eatery was confirmed to be the source of a norovirus outbreak. The county declined to name the restaurant.

Famous Danish eatery Noma, located in Copenhagen and named "best restaurant" for three consecutive years by Restaurant magazine, was the site of a norovirus outbreak in February. More than 60 diners were sickened at this two-star Michelin restaurant where the tasting menu costs $261.

"Even in the best of circumstances, things can go wrong," Kasirye said. "Most restaurants follow certain regulations to reduce the chance of foodborne illness, but in any situation with a lot of people together, the risk is there."

Following the outbreak, the reservations list and menus over the suspected outbreak days were provided to the county by Mulvaney. The county has since contacted these diners, who were asked to complete a questionnaire about what they ate and drank. The county also reviewed the restaurant's handwashing and sanitation practices.

"We disinfected the whole restaurant," Mulvaney said. "Any of our employees who seemed under the weather were sent home. This is a big issue. We reached out to the county and said, 'What can we do?' "

If a norovirus outbreak is confirmed for Mulvaney's, the county may require a further review of its sanitation practices and provide more education for management and employees, according to Kasirye.

"We want to make sure this doesn't happen again," Mulvaney said.

Call The Bee's Chris Macias, (916) 321-1253. Follow him on Twitter @chris_macias.

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