(8:45 a.m. EST) -- As reported on Cruise Critic, Celebrity Cruises' Charleston-based Celebrity Mercury experienced a relatively unprecedented bout of sick-ship-syndrome with three successive voyages resulting in at least 10 -- and in some cases 20-plus -- percent of passengers becoming ill. Norovirus has been confirmed in the first two instances and is suspected in the third.

Norovirus is actually a term for a group of extremely common viruses easily passed from person to person -- and by no means limited to cruise ships. Norovirus affects schools, hospitals and shopping malls -- any place where people gather in relatively large numbers.

If Norovirus is uncomfortable for those who contract it (including yours truly), it's almost never fatal. The medical industry has offered advice on the best ways to avoid it, which include washing hands thoroughly and often (especially after bathroom visits and before meals) and avoiding touching one's face. Sanitizers, typically on offer all over cruise ships (from gangway entrances to restaurants), may also help.

As a result of the consistently high number of passengers impacted over a trio of Celebrity Mercury voyages, Celebrity decided to end the ship's most recent sailing -- during which 369 of 1,829 passengers (20.17 percent) reported symptoms of gastrointestinal illness, according to the CDC -- a day early. (Mercury returns to Charleston today, Thursday, March 18.) It also delayed its next voyage, originally scheduled to leave on Friday, until Sunday, March 21. The extra time will be used to perform an unusually intense sanitization of the ship. "This is a big step for us to take the ship out of service for three days," Celebrity Cruises' President & CEO Dan Hanrahan told Cruise Critic.

We asked him to explain what his team of staffers and crew would do to eradicate the spread of Norovirus onboard.

Cruise Critic: How does Celebrity normally combat the spread of Norovirus?

Dan Hanrahan: As soon as we see any pattern of Norovirus, even minimal impact, we start an enhanced cleaning process. We bring out the cleaning fluids that specifically kill Norovirus and we start to use chlorine. We consider this our "Code Yellow" stage. If we see more than six cases in six hours, we go to "Code Red."

At that point we don't let people handle utensils, we're cleaning at a more frequent rate throughout the ship, and we ask guests to stay in the room 24 hours after the last symptom. You are contagious until that point. If passengers follow that, we refund money for the time they're quarantined.

If they don't follow it, we do not refund the per diem. And yes, it's pretty easy to figure out which passengers respect the contagion period -- and which do not.

Our local medical team is already onboard (they got on in St. Kitts) and there are additional doctors and nurses on the ship.

You'd be surprised at how detailed are the recommendations for cleaning when there's a Code Red situation. In one specific instance, our crew must use one rag to put on disinfectant and then use another rag to wipe it off.

Ninety-nine times out of 100 we get this under control right away. But Mercury's experience is an anomaly.

CC: Have you ever seen such an epidemic of Norovirus on cruise ships before?

DH: Back in 2001, we saw some pretty aggressive viruses. The difference is that today we've got much better recording devices -- we track the spread of the virus much better than before. Back then, it wasn't unprecedented for a cruise line to take a ship out of rotation for an entire cruise in order to sanitize it.

CC: What will be different about the cleaning process than Celebrity's usual Code Red response?

DH: By bringing the ship in one day early -- and delaying its departure by two days, we'll have 78 hours to do a full cleaning. In this case we're bringing on 50 additional crewmembers (some are breaking their holidays, others we're borrowing from other ships). We've hired a carpet cleaning company -- up to a dozen of its staffers will steam clean every carpet on Mercury. We're going to use Bioglobe's electro-static sprayers. This is a new technology and it works a little bit like the spray guns you use to paint cars. In this case you go into a room and spray. This is effective because they're able to venture into hard to reach spots (under tables, a chair arm, inside the closet, on the hangers). It's really effective.

We have hired a retired CDC vessel sanitation staffer to provide guidance and counsel. We also have six of our shoreside management team to provide more oversight to the cleansing process.

CC: With all these extra workers, why do you need three-plus days to clean the ship?

DH: Giving us a few extra days allows us to go through the process of cleaning two more times! It's important to understand that the incubation period of Norovirus is 36 hours, so it's possible that a ship could experience two incubation periods in that time. This way, effectively, the virus dies twice.

We're hiring 30 workers who will clean the port terminal after everyone leaves the ship Thursday. Other than that, we have talked with the Centers for Disease Control, which likes our plan. The CDC has been very good partners in helping us to figure out how to get rid of this.

CC: How will you ensure that this period of sanitizing Mercury will work and that passengers on its cruise departing Sunday won't be affected?

DH: We'll start out in Code Red. Passengers won't be able to serve themselves from the buffet, sanitizers will be everywhere, and we'll be encouraging all to wash their hands. Even though this ship will be spotless (I don't know how it would be possible for Norovirus to live after the three day break), chances are that someone could bring it on, and that's reality. So we'll start the ship off in full Code Red, and we will ask people to respect their fellow guests, to wash their hands with soap and water, and if they do get ill, to honor the quarantine period.

We've also made an offer to anyone who, booked on Sunday's cruise out of Charleston, is nervous about contracting Norovirus. We've given these passengers the option to cancel their cruises and receive a full refund, plus a 15 percent credit toward a future voyage. So far, about eight percent have taken us up on it.

CC: Is Norovirus a "cruise ship illness"?

DH: No, as noted before it's by no means limited to people who travel on cruise ships. The message I always try to give is this: when we are reporting to the CDC the number of passengers affected on a particular voyage, we also look at crew members who have been ill. They live in closer quarters than do our passengers -- and of course they're working in public spaces onboard. And you'll rarely see a significant percentage of crewmembers affected on the voyage. That's because they're trained to follow proper procedures.

Editor's Note: Indeed, according to the CDC, on Celebrity Mercury's March 8 voyage, 20 percent of passengers were ill from a virus that's presumed Norovirus; fewer than 2 percent of crewmembers were affected by the illness.

Tell us what you think!

--Hanrahan's interview was conducted by Carolyn Spencer Brown, Editor in Chief

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