TAMPA, Fla. —  A total of 100 people aboard a cruise ship fell ill a gastrointestinal illness, Royal Caribbean Cruises Ltd. said.

The company said 89 of the people who got sick were guests, and the rest were crew members on the Celebrity Cruises ship called Zenith. The ship was carrying 1,363 guests and 613 crew members.

The ship left the Port of Tampa on April 2 and returned Friday as scheduled.

All the sick passengers were treated onboard with over-the-counter medication, The Tampa Tribune reported. Nearly all the passengers who got sick had improved by the time the ship docked in Tampa, the Miami-based company said.

The illness was thought to have been caused by a norovirus, the company said.

According to the Centers for Disease Control and Prevention, noroviruses are a group of viruses that cause nausea, vomiting, diarrhea and stomach cramps. People infected with these viruses also can experience fever, chills or muscle aches. Symptoms usually last one or two days.

Telephone messages left at the offices of Royal Caribbean Cruises were not immediately returned Saturday.