— -- More than 200 passengers reportedly have been sickened on two different cruise ships, according to the U.S. Centers for Disease Control and Prevention, prompting the agency to take action.
CDC epidemiologists and health officers have been dispatched to assess Royal Caribbean's Legend of the Seas ship and Celebrity Cruises' Celebrity Infinity ship to review the crews' responses to the outbreaks. Both ships were to be inspected at dock in San Diego.
At least 116 passengers and crew members on Legends of the Seas reported gastrointestinal symptoms, including vomiting and diarrhea, during a two-week cruise, according to the CDC.
Royal Caribbean, in a subsequent statement, said the number of those sickened had risen to 133 and the cause was likely the common norovirus, a notorious cause of gastrointestinal distress that spreads easily in the close quarters of cruise ships.
The ship will have "super-sanitization" cleaning when it reaches port to prevent further outbreaks or a spread of the disease as guests disembark, according to a statement released by Royal Caribbean officials.
"Royal Caribbean is undertaking direction from the CDC to inhibit the sickness and prevent further outbreaks, including increasing cleaning and disinfection procedures [and] making announcements to notify onboard passengers and crew of the outbreak, encourage case reporting, and encourage good hand hygiene," read a statement from company officials.
The CDC cited norovirus as the cause of a second outbreak aboard the Celebrity Cruises' Celebrity Infinity ship that the CDC said reported 112 passengers and crew sickened by the virus. During a two-week cruise, at least 106 out of 2,117 passengers reported symptoms including vomiting and diarrhea, according to the CDC.
However, a later statement released by Celebrity Cruises reported just 100 passengers and crew became ill due to the norovirus.
"During the sailing, we took a number of steps to prevent the transmission of the illness, including implementing enhanced cleaning procedures and protocols, and using special cleaning products and disinfectants that are proven to kill norovirus to clean throughout the ship," read a statement from Celebrity company officials.
The crew took immediate steps after the outbreak occurred, the CDC added, including additional cleaning and disinfection immediately after the outbreak started, announcing the outbreak to other passengers so they could take protective steps, and consulting with the CDC.
A CDC epidemiologist and health officer have already boarded the ship, which arrived in San Diego Monday, to conduct an environmental health assessment and review crew procedures.
The norovirus is one of the most common viruses in the United States and the most common cause of acute gasteroenteritis in the country, according to the CDC. Infected cruise ship passengers make up a fraction of the the 19 to 21 million sickened by the virus in the U.S. every year. According to statements from both Celebrity Cruise and Royal Caribbean, norovirus on cruise ships account for one percent of all reported outbreaks of the virus.
In spite of the two recent outbreaks, officials from the Cruise Lines International Association maintained that norovirus outbreaks aboard cruise ships are rare, with just nine reported outbreaks in 2014.
“According to CDC statistics, 20 million people on land in the U.S. come down with norovirus every year -- 1 in 15. The odds of contracting norovirus in a cruise ship outbreak is about 1 in 12,000,” the association said in a statement sent to ABC News.
“Cruise ship crews are keenly focused on keeping ships clean and safe, and their efforts are working to keep passengers healthy,” read a portion of the statement. “The process starts even before a ship leaves port with the screening of passengers for any illnesses they may have contracted on land.”
However, Norovirus can be a problem for cruise ships because of how easily the virus spreads in close quarters, according to infectious disease expert Dr. William Schaffner.
Schaffner, of Vanderbilt University Medical Center, said sick passengers may not show symptoms for days, meaning they can easily spread the virus by touching common surfaces before realizing they are ill. As a result, even the best sanitation efforts by a ship crew may not be enough to stop all norovirus outbreaks.
"They’re just the place where the circumstances are so unusual," said Schaffner. "So many people have such close contact with each other over a prolonged period of time."
Schaffner said the virus can also be spread through close contact and, in rare cases, the virus can travel through the air and infect a person if someone vomits and another person is nearby.
However, there are steps that can lower the risk of infection, Schaffner said.
Use "meticulous hand hygiene," said Schaffner, and, "Samaritan instincts notwithstanding, stay away from people who are sick."