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Nearly 100 passengers on the cruise ship Celebrity Infinity have been sickened by a stomach bug. Above, the 1,033-foot Celebrity Equinox passenger ship, run by the line Celebrity Cruises, sails on the Ems river between Papenburg and Leer in northern Germany June 20, 2009. Reuters/Morris Mac Matzen

Nearly 100 people on a cruise ship are sick with a stomach bug, the U.S. Centers for Disease Control and Prevention said. The Celebrity Infinity ship's crew has increased its cleaning and disinfection efforts in response and is scheduled to dock Tuesday in San Diego.

On the ship, 95 out of about 2,100 passengers -- about 4.5 percent -- were suffering from vomiting and diarrhea, the CDC said. Five out of 964 crew members had also taken ill. The CDC said it would conduct testing to determine cause of the illness, currently unknown. After the ship run by the line Celebrity Cruises docks in San Diego, the company is expected to "complete a thorough public and accommodation super-sanitization cleaning and disinfection," the CDC said, and it is taking steps to prevent other passengers from catching the bug.

In February, the ship Celebrity Equinox, also run by Celebrity Cruises, had an outbreak of the norovirus, which has symptoms much the same as the ones seen now on Celebrity Infinity. In that instance, 142 out of 2,896 passengers, or 4.9 percent, took ill. The company that owns Celebrity Cruises, the Royal Caribbean Cruises, also had an outbreak of norovirus on another of its ships, Grandeur of the Seas, in January.

Outbreaks of contagious illnesses, including norovirus, are often associated with cruise ships, but as the website CruiseCritic pointed out, that's mainly because illness tracking is required on ships, unlike at resorts, and so more outbreaks are reported. The site noted, "Norovirus spreads swiftly wherever there are many people in a small area."

Celebrity Cruises describes itself as a luxury line and an industry leader that provides "an unmatchable experience for vacationers' precious time."