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A children’s hospital in the District has reported an outbreak of norovirus, saying three of its patients showed signs of the illness last week and have tested positive.

Officials with the HSC Pediatric Center, formerly the Hospital for Sick Children, said they have been working closely with the D.C. Health Department to contain the spread of the virus, which produces symptoms commonly associated with stomach flu.

As of late afternoon Sunday, no additional cases had surfaced for 72 hours, said Nathaniel Beers, president of HSC Health Care System, which includes the hospital, located in Northeast Washington. All are being monitored for signs and symptoms, he said.

“We have been successful in keeping it contained,” Beers said.

Beers said he could not release information about the ages of those affected, but the hospital — with more than 20 patients in its care — provides longer-term care for children and young adults through age 21.

Its patients are more fragile and at higher risk of complications, he said, but more serious problems have not developed.

“We’ve not seen any situation that we are concerned will escalate into extreme complications,” Beers said.

It is not clear what caused the outbreak, but the hospital put in place extra safeguards, according to Beers and a memo sent to the hospital’s staff. Officials said the protocols are based on federal guidelines for containing the spread of norovirus.

No children under 12 may visit, nor anyone with norovirus symptoms such as diarrhea and vomiting. Any staff members who develop symptoms must be off the job until the symptoms are gone for at least 48 hours.

No staff members have tested positive for norovirus, but some have complained of stomach distress and are not at work, Beers said.

Outbreaks of norovirus are common, according to the U.S. Centers for Disease Control and Prevention, with the virus spreading easily and quickly through contaminated foods and surfaces.

The most common symptoms are diarrhea, vomiting, nausea and stomach pain. Other possible symptoms include headache, body ache and fever, according to the CDC.

Federal officials say there are 19 million to 21 million cases of norovirus a year in the United States, and it contributes to thousands of hospitalizations.