Over a quarter, or 28.9%, of passengers fell ill on the National Geogrpahic Sea Lion ship operated by Lindblad Expeditions. The percentage was so high due to the small number of passengers onboard: 17 of 61 travelers became ill onboard.

A quarter of all passengers fell ill onboard Fred Olsen’s Balmoral cruise ship. Two hundred and thirty-eight of 1,163 total passengers fell ill during the 30-day voyage.

Almost a quarter, or 22%, of all passengers fell ill on Celebrity Cruises’ Mercury ship. That happened twice in 2010. The first time 293of 1,163 cruise fell ill on a 30-day journey. The second time 411 of 1,833 cruisers fell ill on an 11-day trip.

Another 22.2% of passengers fell ill on Holland America’s Ryndam cruise ship. That’s 272 of 1,223 total passengers on a 16-day journey. Twelve outbreaks have occurred on the Ryndam, the most on any one ship in the cruising industry.

One-fifth, or 20.6%, of passengers fell ill on the Royal Caribbean International’s Explorer of the Seas this January. That’s 634 passengers out of a total 3,071 guests onboard. The incident was one of three virus outbreaks reported in January 2014.

There have been multiple norovirus outbreaks on cruise ships over the last month, but how big of a concern is this for potential cruisers?

In the past ten years, only nine cruise virus outbreaks have caused more than 20 percent of passengers to become ill. This according to data reported the CDC.

The worst case was in 2008 onboard the American Canadian Cruise Grand Caribe when almost half, or 43 percent, of passengers became ill.

There were only 63 passengers onboard which explains the high percentage, but no other outbreak in the past twenty years has crossed the 30 percent threshold in that time period.

There were less than 150 passengers onboard during four of nine worst outbreaks, leading to shockingly high percentages.

However, it’s the outbreak was likely even more evident since passengers more easily noticed how many of their fellow cruisers became ill.

Double Trouble

Two of the worst cruise ship outbreaks in the past decade happened onboard Celebrity Cruises Mercury.

Approximately 22 percent of passengers fell ill on the ship on two separate voyages in 2010. The first time 293 of 1,163 cruisers were reported ill on a 30-day journey. The second time 411 of 1,83 cruisers were reported ill on a 11-day trip.

Below is statistics on the nine cruisers where more than 20 percent of passengers fell ill during a virus outbreak:

YearCruise LineShipCruisers Reported Ill (%)Cruisers Reported Ill (#)Total Cruisers (#)CauseLength of Voyage
2008American Canadian Cruise LineGrande Caribe43%2968Norovirus12
2011Lindblad ExpeditionsNational Geographic Sea Lion28.87%1761Unknown7
2010Fred Olsen Cruise LinesBalmoral25.19%2931163Unknown30
2010Celebrity CruisesMercury22.43%4111833Norovirus11
2007Cruise WestSpirit of Nantucket22.22%836Unknown7
2005Holland America LineRyndam22.2%2721223Norovirus16
2010Celebrity CruisesMercury22.1%4061829Norovirus11
2014Royal Caribbean Cruise LineExplorer of the Seas20.6%6343071Unknown10
2003Windjammer Barefoot CruisesLegacy20.54%22107Specimens not obtained7
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Photo Credit: The Royal Caribbean International's Explorer of the Seas is docked at Charlotte Amalie Harbor in St. Thomas, U. S. Virgin Islands, Sunday, Jan. 26, 2014. Thomas Layer / AP Photo