All four ships belonging to the Disney Cruise Line passed inspections by the Vessel Sanitation Program with a 100% score, making it the only cruise line with a perfect rating for their entire fleet. Inspections are held twice a year, randomly, and apply to any ship that enters/leaves a U.S. port throughout the year.
The Vessel Sanitation Program conducts these inspections to gauge just how well the ships are operating and if they are complying with sanitation standards. They use a 44 item long USPH (United States Public Health) checklist; to score a 100%, ships need perfect marks for each item. A failure would be a score of 85% or below.
All major areas of each vessel fall under the inspection, including “medical facilities, portable water systems, swimming pools and hot tubs, galleys and dining rooms, child activity centers, hotel accommodations, ventilation systems, and common areas of the ship.”
Only ships that enter/leave U.S. ports underwent inspections, so not every ship for each cruise line was factored in. That being said, Disney appears to be the only cruise line with a company-wide perfect score.
Source: Cruise Fever