A new study on illnesses aboard cruise ships indicates that there may be many more sick people on board than official reports would lead us to believe.

The U.S. Centers for Disease Control and Prevention (CDC), in Atlanta, recently published a report in the journal Clinical Infectious Diseases. It found that in a closely monitored case of a norovirus outbreak on a cruise ship with 1842 passengers in 2009, that 40% of the people that got sick never reported their illness to the ship’s infirmary.

Norovirus causes acute gastroenteritis, according to the CDC. It causes vomiting, and stomach pain. They report that it is “recognized as the leading cause of foodborne-disease outbreaks in the United States.”

CDC staffers that boarded the ship during the outbreak found that 15.4% of the cruise ship’s passengers became ill – but only 60% of the sick passengers informed the ship’s staff that they were sick.

The researchers noted that passengers that did not report the illness said either they did not feel that bad or had their own medications. Of concern was that some said they did not report their illness because they did not want to be ordered into isolation while on vacation – a problem for cruise ships that want to control infectious illnesses