More people experienced gastrointestinal illnesses while aboard a cruise ship in 2024 than any other year since 2012, according to data from the Centers for Disease Control and Prevention (CDC).

The CDC reported that there were 16 outbreaks of stomach illnesses aboard cruise ships last year, which affected 1,894 passengers and 245 crew members. The data shows that there were also 16 outbreaks of stomach bugs in 2012 but indicated that illnesses on board had plunged during the COVID-19 pandemic.

The drop took place after the cruise ship industry shut down for more than a year and then restarted with limited capacity and more cleaning protocols in place, the Washington Post reported.

Five of the cruise ship outbreaks took place in December alone, according to CDC data. Those outbreaks caused 781 passengers and 109 crew members to become ill, with most of those experiencing diarrhea and vomiting.

In three of the five outbreaks, norovirus — the highly contagious virus that causes stomach illnesses — was the main culprit, officials reported. CDC data shows that of the 16 outbreaks, 14 were caused by norovirus while the other two were brought on by E. coli and salmonella.

The CDC also noted that norovirus cases have continued to increase around the U.S. The agency said that most cases of the virus are seen between November and April. However, federal health officials warn that cases of the virus can be seen at any time.

On an annual basis, norovirus causes an average of 19 million to 21 million illnesses and more than 100,000 hospitalizations, according to the Post. Health experts told the outlet that cases of the virus can become more prevalent in crowded, semi-enclosed environments that are often found on cruise ships.

The CDC found that from 2006 to 2019, an average of 12 outbreaks of gastrointestinal illnesses took place on cruise ships.