(9:30 a.m. EDT) -- The month of March has seen an uptick in the number of gastrointestinal illnesses reported on cruise ships.

According to the U.S. Centers for Disease Control and Prevention Web site, an outbreak on Royal Caribbean's Vision of the Seas affected 121 out of 2,756 passengers and crew on an 11-night Caribbean sailing that ended March 8. A second outbreak in March, on Princess Cruises' Ruby Princess, affected over six percent of passengers and crew -- or 276 out of a total ship population of 4,300 -- on a 7-night Caribbean cruise.

Most recently, 84 passengers reported symptoms over the course of 21 days onboard Cunard's Queen Elizabeth.

Local Los Angeles news station KTLA 5 quoted Cunard as saying four percent of the 1,900 passengers on the five-week tour of the South Pacific became ill, roughly the same percentage as onboard Vision of the Seas. A spokesperson for Cunard told Cruise Critic that by the time the ship reached port in Los Angeles no passengers were reporting symptoms. And, after a thorough cleaning of the ship while in port, only one passenger onboard has reported symptoms since. The ship has filed a report with the CDC, however.

Cruise lines participating in the CDC's Vessel Sanitation Program -- and every major operator does -- are required to report the total number of GI cases evaluated by the medical staff before the ship arrives at a U.S. port when sailing from a foreign port. A separate notification is required when the count exceeds 2 percent of the total number of passengers or crew onboard. The CDC mandates a standardized protocol of reporting and cleaning procedures during and following an outbreak onboard. The organization then completes its own investigation to attempt to determine the cause of the illness.

--by Jamey Bergman, U.K. Production Editor