So far this year, The Centers for Disease Control (CDC) has awarded 11 ships perfect scores and assigned failing grades to two more.

The CDC's surprise cruise ship cleanliness exam is conducted twice a year, with an 86 considered a passing grade; a 100 is a perfect score. All passenger ships that call at any U.S. ports are required to undergo the process. The CDC has inspected 55 ships since January 1, 2015.

Those that received perfect scores are: Carnival Miracle and Carnival Pride; Celebrity Infinity; Holland America Line's Eurodam, Noordam and Ryndam; Island Princess and Ruby Princess; Norwegian Jewel and Norwegian Sky; and Seadream I of Sea Dream Yacht Club.

Silversea's Silver Shadow and Caribbean Fantasy, run by America Cruise Ferries, received failing grades.

Silver Shadow received a score of 95 when it was inspected in January, but in its subsequent inspection April 16, the 382-passenger ship received an 82. Among the issues cited in the report: inadequate sneeze guards at the buffet, uneven temperatures for stored food and inadequate disinfection of whirlpools in cabins.

In 2013, Silver Shadow crew were cited for hiding food, equipment and dishware from CDC health inspectors during an inspection. The ship had passed each inspection since.

The line put out a statement about the most recent score. "Silversea is deeply disappointed by this unsatisfactory score and takes very seriously its responsibilities for the health and safety of its guests and crew.  All Silversea ships have comprehensive policies and rigorous training programs in place to make certain its staff and crew implement best practices onboard. Silversea has taken measures to ensure that Silver Shadow immediately reestablishes the highest standards in all areas of its operations."

Ships that fail a CDC inspection are required to submit a corrective action report. They are then re-inspected within a "reasonable time period."

Caribbean Fantasy is run by America Cruise Ferries, making trips between Puerto Rico and the Dominican Republic. The ship received a score of 84 during its inspection January 23, 2015.  The report cited items including improperly cleaned stove-tops and a substance that appeared to be mold on the ice shields.