The U.S. Centers for Disease Control and Prevention has announced that an unknown illness sickened several people aboard a cruise ship earlier this month.

This incident is one of 18 gastrointestinal outbreaks on cruise ships this year that met the CDC's threshold for public notification.

More than 140 people aboard Royal Caribbean's Navigator of the Seas suddenly got sick with vomiting and diarrhea during a round-trip cruise from Los Angeles to Mexico. The outbreak affected 134 passengers and seven crew members after the ship departed from LA on July 4.