With more than 22 million passengers each year, cruise ships are a popular way to get out and see the world, but they’re also a great venue for spreading contagious foodborne illnesses to a large number of people in a short amount of time, according to a new study on the health and safety of the cruise ship industry by ProPublica. Last year, more than 1,700 illnesses were confirmed among cruise ship passengers and crew. The majority of those illnesses were the result of norovirus, a vomit-inducing bug that’s easily transmitted via food, water, and person-to-person contact. Several factors contribute to the likelihood of a norovirus outbreak occurring on a cruise ship. For one, a large number of people are living in close proximity for days or weeks at a time. It only takes one or two contagious individuals to begin a domino effect through the thousands of passengers aboard a ship. Combine that with the fact that passengers often share buffet lines, bathrooms and other items in common, and you have a potential perfect storm for infectious disease. Granted, norovirus outbreaks happen everywhere, and, in reality, only one percent of norovirus illnesses originate on a cruise ship. But cruise ship outbreaks are easier to pinpoint because the patients are confined in close proximity. It’s more difficult to trace a norovirus outbreak back to something such as a contaminated airplane bathroom because all of the passengers have dispersed before symptoms kick in. The U.S. Centers for Disease Control and Prevention keeps tabs on all outbreaks on cruise ships through its Vessel Sanitation Program. Nine cruise ship outbreaks have occurred each year for the past three years, with all but two of them being caused by norovirus. The cause of the other two? E. coli. Twice a year, a CDC team inspects each vessel that docks in the United States. Only a few ships have failed inspection in recent years, according to ProPublica, but ships that pass their inspections have still been tied to outbreaks. Some of the most common food safety deficiencies on ships involve dish trollies, which often carry improperly stored food or leftover grease and food debris. Sick kitchen workers are also a high-risk source of contamination. Since CDC began tracking information on cruise ship outbreaks 20 years ago, more than 200 outbreaks have been associated with cruises.