MIAMI - Stomach virus outbreaks on cruise ships ballooned this year to 23 occurrences on 19 ships - triple last year's number and exceeding those of the past four years combined, a report released yesterday by the Centers for Disease Control shows.

Since October, more than 1,200 passengers and crew on board eight cruises have been stricken with gastrointestinal illness - many of whom sailed from South Florida ports.

And while ships that have been taken out of service for disinfection - such as Holland America's Amsterdam and Disney's Magic - have reported no outbreaks on subsequent cruises, there is no way to know whether additional ships will report high levels of the bug.

"We would hope on a daily basis to say there are no outbreaks and that the outbreaks are over," said Dave Forney, chief of the CDC's vessel sanitation program. "Unfortunately, we can't say that, just like we couldn't predict the outbreaks we've had in the past few months."

The CDC's report "emphasizes that it is very easy for the virus to be transmitted from person to person, especially in an enclosed environment like a cruise ship," said Dr. Elaine Cramer, a medical epidemiologist with the CDC, who wrote the report.

The report found that illness on subsequent cruises with new passengers - or even on ships of the same cruise line - "suggests that environmental contamination and infected crew members can serve as reservoirs of infection for passengers."

The fecal/oral virus, which passes from hand to mouth, spreads quickly, as passengers touch the same handrails and elevator buttons and use the same public restrooms. Passengers are advised to wash their hands frequently. Thorough cleaning with a chlorine-based solution is required on ship surfaces to kill the virus.

Symptoms of the bug, known as Norwalk-like or norovirus, include nausea, vomiting, diarrhea, abdominal cramps, fever and headache. The illness, which can be dangerous for the elderly and those with medical problems, affects an estimated 23 million people in the United States each year.

Cramer cautioned, however, that the increased number of cruise line cases in 2002 does not take into consideration any growth in the number of passengers who have taken cruises and the length of the voyages.

During the first nine months of this year, 11.3 percent more passengers cruised from the North American market, compared to the same period of 2001.