are getting safer over time. The study claims that, even as capacity increased 55 percent between 2009 and 2018, the number of overall "operational incidents" declined 37 percent and the rate of man-overboard cases dropped 35 percent. Disease Norovirus Norovirus is a virus that commonly causes gastroenteritis in developed countries, and is also a cause of gastroenteritis on cruise ships. It is typically transmitted from person to person. Symptoms usually last between 1 and 3 days and generally resolve without treatment or long term consequences. The incubation period of the virus averages about 24 hours. Norovirus outbreaks are often perceived to be associated with cruise ships. According to the United States CDC, the factors that cause norovirus to be associated with cruise ships include the closer tracking and faster reporting of illnesses on cruise ships compared to those on land; the closer living quarters that increases the amount of interpersonal contact; as well as the turnover of passengers that may bring the viruses on board. However, the estimated likelihood of contracting gastroenteritis from any cause on an average 7-day cruise is less than 1%. In 2009, during which more than 13 million people took a cruise, there were nine reported norovirus outbreaks on cruise ships. Outbreak investigations by the United States Centers for Disease Control and Prevention (CDC) have shown that transmission among cruise ship passengers is primarily person-to-person; potable water supplies have not been implicated. In a study published in the Journal of the American Medical Association, the CDC reported that, "Perceptions that cruise ships can be luxury breeding grounds for acute gastroenteritis outbreaks don't hold water. A recent CDC report showed that from 2008 to 2014, only 0.18% of more than 73 million cruise passengers and 0.15% of some 28 million crew members reported symptoms of the illness." Ships docked in port undergo surprise health inspections. In 2009, ships that underwent unannounced inspections by the CDC received an average CDC Vessel Sanitation Program score of approximately 97 out of a total possible 100 points. The minimum passing inspection score is 85. Collaboration with the CDC's Vessel Sanitation Program and the development of Outbreak Prevention and Response Plans has been credited in decreasing the incidence of norovirus outbreaks on ships. Legionnaires' disease Other pathogens which can colonise pools and spas including those on cruise ships include Legionella, the bacterium which causes Legionnaires' disease. Legionella, and in particular the most