(Updated 4:45 p.m. EST) -- The number of cruise passengers stricken with gastrointestinal illnesses, most notably Norovirus, fell dramatically from 2017 to 2018 according to
reported by the Centers for Disease Control and Prevention. While both years documented 11 outbreaks, 2018 resulted in only 575 affected passengers as opposed to 1,296 the previous year.Cruise ships are required to report any instances of gastrointestinal illness through the CDC's Vessel Sanitation Program (VSP); health officials are not required to track hotels and resorts. The VSP generally posts news of outbreaks if they meet the following criteria: the ship's itinerary involves a U.S. port; the cruise spans three to 21 days; the ship is carrying 100 passengers or more; and 3 percent or more passengers and crew report active symptoms to the ship's medical staff.Symptoms of Norovirus, the most common gastrointestinal illness reported on cruise ships, are similar to those of the stomach flu, such as nausea, vomiting, diarrhea and abdominal cramps as well as a possible low-grade fever and headache.So far in 2019, the CDC has reported one gastrointestinal illness outbreak, on Royal Caribbean's Oasis of the Seas. The cruise was cut short last week after 277 people onboard fell ill; the final total of passengers affected was 561. All passengers received full refunds of their cruise fare paid.Learn more about Norovirus in our comprehensive Q&A, and read our five reasons why you should worry less about it on your cruise.