More than 250 passengers and crew members on board the first cruise ship to dock in Portland this season were sickened by the norovirus during their voyage, according to the U.S. Centers for Disease Control and Prevention.

The Balmoral arrived Sunday morning at the Portland Ocean Terminal, the first of 76 cruise ships that are expected to visit the city this year. The Balmoral had 919 passengers and 520 crew members when it sailed into Portland Harbor.

But only six passengers had to be quarantined in their cabins Sunday, a standard procedure when ships arrive in port with ill passengers or crew.

“The reality is that many of the ships that come into port have cases of the norovirus (some more severe than others),” the city’s spokeswoman, Jessica Grondin, said in an email to the Portland Press Herald. “It just doesn’t often get reported. The ship’s staff typically notifies our port security director upon arrival when there are large volumes of passengers with the virus.”

Norovirus is a gastrointestinal disease – not uncommon on cruise ships – that can survive on surfaces for at least a week. Only bleach-based cleaners can eradicate it. It is highly contagious.

Symptoms – which include vomiting, diarrhea, fever, cramps, headache and chills – usually last a few days and end quickly, but an infected person can continue to infect others for up to three days after recovery. People can be infected by another person, contaminated food or water or by touching a contaminated surface.

The CDC reported on its website that 252 – or 27.42 percent – of the passengers on the Balmoral and eight – or 1.54 percent – of the crew members had been sickened by the norovirus during the voyage, which began April 16 in Southampton, England.

Grondin said in an email that only six of the passengers were still sick when the ship docked Sunday in Portland and therefore had to be confined to their cabins.

The cruise ship’s owner, Fred. Olson Cruises, said in a news release that its standards for isolating sick guests in their rooms exceed CDC standards. Instead of 24 hours, guests must remain in their rooms for 48 hours.

In addition, Grondin said, “they are very diligent about wiping down handrails and hard surfaces as well as insisting and strongly encouraging all passengers to use hand sanitizer when embarking and disembarking the ship.”

MAYOR VISITED SHIP, FELT SAFE

The CDC has been monitoring the Balmoral since it departed on a monthlong cruise that took it from England and up the East Coast of the United States.

Two CDC Vessel Sanitation Program environmental health officers and an epidemiologist boarded the ship when it arrived April 30 in Baltimore, the CDC reported on its website. The team collected specimens and conducted a rapid test for norovirus that came back positive. The specimens will be sent to the CDC for additional testing.

Mayor Ethan Strimling welcomed the first passengers and crew members to the city when they arrived at 7:30 a.m. Sunday. Strimling said he learned about the outbreak just minutes before boarding the ship for a tour.

“I could see there were people wiping things down constantly. We got onto the elevators and they wiped the elevators down,” said Strimling, who met with the ship’s captain.

He said he was not concerned about getting sick.

“It felt safe and secure,” Strimling said.

Toni Doucette, facility security officer for the city of Portland, who was responsible for the vessel while it was in Portland Harbor, said she was told Sunday there were six passengers exhibiting symptoms and they had been isolated to their cabins.

Doucette said she was told the crew was sanitizing the ship regularly, closing off public restrooms and asking passengers to use their cabin restrooms instead.

Doucette said the foggy, damp and chilly weather had kept many people on board. She said only about half of the passengers left the ship to visit Portland.

Clay Holtzman, a spokesman for Maine Medical Center, said there have been no reported widespread outbreaks of norovirus originating on a cruise ship docked in Portland in recent memory that were treated by the hospital’s infectious disease team.

OUTBREAKS RELATIVELY INFREQUENT

According to the most recent data posted on the CDC’s website, outbreaks such as the one that occurred on the Balmoral are relatively infrequent, but still occur on occasion. There have been nine outbreaks of norovirus on board cruise ships monitored by the CDC this year.

From 2008 to 2014, the CDC said 74 million passengers sailed on ships monitored by its Vessel Sanitation Program and 129,678 became sick from an acute gastroenteritis-related illness. Of that total, only one in 10 – or roughly 13,000 passengers – became ill from norovirus.

The CDC says cruise ships under its Vessel Sanitation Program’s jurisdiction are subject to two unannounced inspections each year. If a ship sails outside the United States for an extended period of time, it may be inspected upon its return to the U.S.

Cruise ships are scored on a 100-point scale, with 100 being perfect. An 85 or below is considered a failing score. If a ship fails an inspection because of an imminent public health risk, the Vessel Sanitation Program can recommend the ship not set sail.

During its last inspection in September 2015, the Balmoral scored 96, according to a report posted on the CDC’s Vessel Sanitation Program site.

The Balmoral is due to return May 20 to Southampton. According to the Fred. Olson Cruises website, the ship was named after the Scottish home of the royal family. It is capable of carrying up to 1,350 guests and is the newest vessel in a fleet of cruise ships owned by the cruise line.

The Balmoral has 710 rooms, a crew of 520 and a length of 218 meters.

Fred. Olson Cruises could not be reached Sunday night for comment, but in an April 29 news release the company said it is cooperating fully with maritime agencies and authorities “and will continue to make every effort possible to ensure the safety and well-being of all its guests and crew on board.”

The Balmoral left Portland Harbor on Sunday afternoon, headed to Saint John, New Brunswick.

More than 100,000 cruise ship passengers and 40,000 crew members will visit Portland between May and October, Grondin said in a news release. The cruise ship activity is in addition to The CAT, which will ferry visitors between Portland and Yarmouth, Nova Scotia, beginning in June.