The Mission District’s Italian eatery Delfina has been considered one of the Bay Area’s best restaurants for more than a decade; Craig Stoll, its co-owner and chef, won a coveted James Beard award in 2008.

But in December, its many accolades could not protect Delfina from an unusual incident. On a night the restaurant was booked solely for a private party, about two dozen patrons were sickened by food poisoning.

The staff determined what each victim ate, and since a vegetarian was among those sickened, oysters, beef tartar and other foods were eliminated as the sources of illness.

“We narrowed things down to the most common denominator,” Stoll said. Their conclusion: Tainted produce, most likely salad greens.

The restaurant contacted its suppliers, but no alert went out to the public, and there was no government investigation. The San Francisco Department of Public Health had not heard of the incident until contacted by The Bay Citizen.

In what appears to be a gap in the food supply safety net, there is no requirement for restaurants to report when their diners are affected by food-borne illnesses even when large numbers of people get sick.

“They are not obligated to report it,” said Richard Lee, director of environmental health regulatory programs for the city.

Mandatory reporting is not required at the state level either, according to the California Department of Public Health. Under both state and local laws, reporting is required only when restaurant workers become sick.

While city health officials have embarked on a high-profile crackdown of popular unlicensed food sales — including the closing last year of the Underground Market and recent action to regulate pop-up dining events like the ForageSF’s Wild Kitchen — the Delfina incident shows that health officials are sometimes unaware of actual cases of food-borne illness.

Rajiv Bhatia, the city’s director of environmental health, said the Delfina incident was now under investigation, but added that it was highly unusual for health officials to be unaware of a case involving so many diners.

“Over the past 13 years, I have not encountered a case where an outbreak of this magnitude was not reported directly by an ill consumer or a medical provider to the department,” Bhatia said.

He suggested a need for stricter rules. “I believe that reporting of potential outbreaks should be mandatory for supermarkets, restaurants, schools and workplace cafeterias, even though this is not a requirement under current law,” he said.

The city has approximately 4,500 licensed food establishments, and reported illness outbreaks are rare.

Nathan Wolfe, a visiting professor in human biology at Stanford University and director of the Global Viral Forecasting Initiative in San Francisco, said that better systems for reporting food-related illnesses were essential.

In his new book, “The Viral Storm: The Dawn of a New Pandemic Age,” Wolfe writes that many illnesses, including viruses, make the jump to humans from animals when used for food, with transmission possible during hunting, butchering or consumption.

A worst-case scenario was portrayed in the recent film “Contagion,” on which Wolfe served as a consultant. Spoiler alert: The movie’s fictional pandemic starts in a restaurant kitchen.

“Early detection can play a huge role” in treating and containing outbreaks, Wolfe said in an interview, but “we still have not largely cracked the problem of biosurveillance.”

An increasingly interconnected and complex food chain makes that task daunting. “With the advent of processed meats,” Wolfe wrote in the book, “a single hot dog at a baseball game can consist of multiple species (pig, turkey, cattle) and contain meat derived from hundreds of animals.” Typical meat eaters “will consume bits of millions of animals in their lifetimes.”

Vegetarians are also at risk since animal waste can taint produce at farms.

The World Health Organization, governments and private companies are developing elaborate efforts to catch outbreaks early, including the monitoring of key words in Google searches — for example, a sudden spike in queries for a symptom — that could pinpoint emerging illnesses in specific regions. Yet here in San Francisco and California, no law requires restaurants even to pick up the phone and report when diners are sickened.

At Delfina, which consistently achieves high scores on health inspections, Stoll said there had not been an illness before or since that night, but he wants the mystery solved.

“We’re not positive what it was,” he said.

This article also appears in the Bay Area edition of The New York Times.