Oysters and clams, if eaten raw, carry a particular risk of passing on diseases such as hepatitis or Norwalk-like viruses.

The outbreaks, of a germ known as a Norwalk-like virus, are creating a mystery for health officials and are rattling passengers and cruise company executives.

Noroviruses are also called Norwalk-like viruses and caliciviruses.

The outbreak was attributed to a Norwalk-like virus (norovirus).

Common stomach flu viruses include norovirus or Norwalk-like virus, adenovirus, rotavirus, calicivirus, and astrovirus.

Norwalk-like viruses, also called noroviruses, are hard to contain, in part because patients remain contagious for up to two weeks after their own symptoms have disappeared.

And, after checking with her doctor, she also took along over-the-counter antidiarrheal medicine, just in case traces of the common but contagious Norwalk-like virus remained.

Soon passengers were vomiting and showing other signs of the Norwalk-like virus, which causes a gastrointestinal ailment that is usually not life-threatening.

Norwalk-like viruses, also known as noroviruses, are a family of very common, highly contagious germs that cause gastrointestinal illness.

But Dr. Murray said that scientists are not sure which disinfectants kill Norwalk-like viruses, and that more research is needed to find out.