Royal Caribbean International gets hit again by a mysterious virus, this time its luxury liner Explorer of the Seas becoming the victim. This is only about a week after one of its ships, Majesty of the Seas, was struck by a strain of norovirus, which left 68 passengers and 84 crew members ill.

In the case of Explorer of the Seas, about 281 passengers and 22 crew members fell ill with bouts of vomiting and diarrhea. The cause of the distress is still unknown, and is currently being investigated by the Centers for Disease Control and Prevention (CDC).

Janet Diaz, spokesperson for Royal Caribbean International, said those affected have been responding well to medication so far, and that the ship itself skipped a scheduled stop to sail directly to San Juan, Puerto Rico, for extensive sanitizing. Diaz also said that they are coordinating very closely with the CDC on this matter.

The ship has now docked at St. Thomas in the U.S. Virgin Islands, where a Vessel Sanitation Program environmental health officer and an epidemiologist of the CDC boarded the ship for investigation, assessment, and evaluation of the outbreak. The CDC said specimens are being collected, and they are to be sent to the CDC lab for analysis.

A week ago, the Majesty of the Seas, was forced to dock in Miami because vomiting and diarrhea left 68 passengers and 84 crew members bed-ridden. It was later found that the ship was infected by a strain of norovirus, a very contagious virus that can be transmitted through contaminated food and water, among others. Also called the Norwalk virus, it has been shown to affect 267 million individuals each year.

The CDC describes norovirus as, "the most common cause of gastroenteritis in the United States. CDC estimates that each year on average 19 to 21 million cases of acute gastroenteritis are caused by noroviruses. That means about 1 in every 15 Americans will get norovirus illness each year. Norovirus is also estimated to cause 56,000 to 71,000 hospitalizations and 570 to 800 deaths each year in the United States." There is no vaccine against norovirus, and no cure for those who are affected by it. Available medication can only treat the symptoms of norovirus. Antibiotics do not work, since they work against bacteria, not viruses.

Prior to the infections reported on these two Royal Caribbean Cruise ships, an even earlier infection was also reported on Royal Caribbean Cruise ship Vision of the Season last February, when 118 passengers fell ill.