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.

The mystery gastrointestinal illness broke out onboard the Navigator of the Seas, a Royal Caribbean ship, on a round-trip to from the US to Mexico. Some 134 of the 3,914 guests fell ill, along with 11 crew members. Symptoms included sickness, abdominal cramps and diarrhoea.