NEW YORK (MainStreet) — Some 600 passengers are heading home early from a cruise due to another virulent outbreak of gastrointestinal illness. The Royal Caribbean ship Explorer of the Seas is the latest bug boat to void a vacation, with nearly 20% of passengers falling ill.

The Centers for Disease Control and Prevention (CDC) tracks these outbreaks and reports nine instances of onboard illnesses occurred in 2013. Cruise ships participating in the agency's Vessel Sanitation Program (VSP) are required to report the total number of gastrointestinal illness cases evaluated by the medical staff before an international cruise ship arrives at a U.S. port.

The CDC says norovirus is the most common cause of acute gastroenteritis in the U.S., resulting in about 21 million illnesses, and as many as 800 deaths. It is also the most-reported onboard ailment on international cruises.

A CDC environmental health officer and an epidemiologist boarded the Explorer of the Seas Sunday to evaluate the outbreak and response activities.

Which cruise lines are the most infected? The list below has been compiled from CDC surveillance reports and investigations and shows the total number of cruise ships experiencing mass outbreaks of illness occurring from 2009 to the present. Voyages were of 3 to 21 days in length, with 3% or more of the passengers or crew reporting symptoms, out of a total of 100 or more passengers.

Per cruise line, here is a list of the most ship outbreaks of mass illness on international cruises (2009-present)

  • Celebrity, 15
  • Holland America, 15
  • Princess, 15
  • Royal Caribbean, 6
  • Cunard Line, 4
  • Norwegian, 4
  • <

  • Carnival, 3
  • Crystal, 2
  • Fred Olsen Cruise, 1
  • Lindblad Expeditions, 1

  • Oceana, 1
  • P&O, 1
  • Prestige, 1
  • Regent Seven Seas, 1

Prior to embarking on a cruise, passengers can see the latest sanitation scores for the carrier or ship of their choice using the CDC Cruise Ship Inspection search tool. A score of 86 or higher represents a satisfactory score, while 85 or below is unsatisfactory. Full details of inspections, with corrective actions taken, are noted in accompanying reports.

The most recent inspection data, dated July 25, 2013, gave the now port-bound Explorer of the Seas a grade of 98, with several deficiencies noted and corrected.

--Written by Hal M. Bundrick for MainStreet