Lab tests confirmed the presence of the virus, which causes nausea, vomiting and diarrhea, said Janice Okubo, spokeswoman for the Hawai'i Department of Health.

The virus infected about 9 percent of the ship's 2,500 passengers, according to NCL. The ship returned to Honolulu on Monday morning after its weekly seven-day cruise around the islands.

"It's one of the common viruses we've been seeing on cruise lines," Okubo said. "Most of the time, people recover."

The Health Department has notified NCL of the test results, she said. But a statement from the company sent Monday makes no mention of the Norwalk virus.

"Pride of Hawaii has been carrying out an extensive range of preventive measures on board and does not expect this issue to carry forward into the next cruise," the statement said.

Passengers who felt sick during the course of the voyage and their cabinmates were asked to remain quarantined in their rooms for 24 hours. NCL said it was giving those passengers a $200 on-ship credit.

The company reported Friday that the outbreak had been confined to just the one ship, and the Pride of Aloha and Pride of America were not affected.

Passengers on the next cruise will be told before boarding that some people had suffered stomach flu symptoms, according to the statement. They will be given the opportunity not to sail if they're uncomfortable after being told about the illness.

Investigators from the U.S. Food and Drug Administration will handle the outbreak, Okubo said.

Cruise lines usually take measures including keeping passengers isolated and rigorously cleaning surfaces on board, she said.

A field supervisor for the U.S. Food and Drug Administration didn't immediately return a phone call seeking comment.

The Norwalk virus typically lasts 24 hours, although it may persist for several days. There's no good way to treat its symptoms besides drinking fluids.

It can become life-threatening for the very old, the frail and the very young if they become dehydrated from the virus.

Additional symptoms include low-grade fever, chills, headache, muscle aches and tiredness.