HONOLULU -- Norwegian Cruise Line said about a sixth of the 2,500 passengers onboard its Pride Of Hawaii cruise ship became sick this week.

The passengers fell ill with the stomach flu after leaving Honolulu Harbor Monday.

NCL said it is telling its passengers booked on its next cruise about the outbreak so they have the option of not sailing if they are not comfortable.

Operators of buses and cabs who handle passengers of the cruise line were notified about the problem on Friday.

NCL sent out a memo saying, "To prevent an occurrence of the Norwalk virus... NCL would recommend that all motor coaches and tour equipment utilized by Pride of Hawaii and Pride of Aloha passengers be sanitized prior to and after each excursion."

On Friday afternoon, NCL said the outbreak has been confined to just the one ship. It said the "Pride of Aloha" and "Pride of America" are not affected.

NCL is not required to notify the state Department of Health when there is an outbreak.

Because of federal jurisdiction its notification extends to the Food and Drug Administration and Centers for Disease Control and Prevention.

State health officials said they were contacted Friday to run lab tests for the company. The tests would confirm if it is the Norwalk virus or some other type of illness.

NCL said it is cleaning and disinfecting all its buses on all islands and is asking all its tour operators to do the same.

Passengers onboard the Pride of Hawaii were not allowed to disembarked in Maui on Wednesday. The company said it was not because of the outbreak; it was because of an oil spill clean up in Kahului.

The ship apparently spent the day at sea and then diverted to Kona. It arrived in Maui on Friday and is expected to continue on to Kauai.

Passengers who became ill with nausea vomiting and diarrhea were asked along with their cabinmates to remain quarantined in their rooms for 24 hours.

NCL said it is giving those passengers a $200 on-ship credit.