There are about 2,500 reported norovirus outbreaks in the U.S. each year. Norovirus, which is sometimes called the "cruise ship virus," causes more than 90% of diarrheal disease outbreaks on cruise ships, according to the CDC. However, norovirus outbreaks on cruise ships account for only a small percentage of all reported norovirus outbreaks.

Among 3,001 passengers aboard the cruise line’s Ruby Princess vessel, 103 reported being ill during a sailing that ended Wednesday, along with a dozen crew members, according to the Centers for Disease Control and Prevention. Their main symptoms were diarrhea and vomiting.