Worried about catching a stomach bug on your next cruise? Here’s the good news: Outbreaks of norovirus and other gastrointestinal illness on cruise ships are on the decline.

The Centers for Disease Control in Atlanta recorded just 15 outbreaks of gastrointestinal illness on cruise ships in 2008, down from 21 in 2007 and 34 in 2006.

The decline, moreover, came even as the number of people cruising continued to rise. The Cruise Lines International Association says the industry carried 13.2 million passengers in 2008, up from 12.6 million in 2007 and 12.0 million in 2006.

Cruise ships arriving in U.S. ports must report all cases of gastrointestinal illness treated by on-board medical staff to the CDC’s Vessel Sanitation Program division, and a separate notification is required when the number of cases of illness on a cruise exceeds 2% of passengers and crew. When the number of cases of illness on a cruise exceeds 3% of passengers and crew the CDC issues a public report.

An analysis of the public reports by USA TODAY’s Cruise Log shows that, as in past years, almost all of the outbreaks in 2008 — 13 out of 15 — were due to norovirus. There was one outbreak of E. coli, on the Pacific Princess in January of 2008. There also was one outbreak that was the result of both norovirus and E. coli (on the Norwegian Dream in April).

Of the 15 outbreaks in 2008, six were on ships operated by Holland America — a line that has had a disproportinate share of the industry’s gastrointestinal illness outbreaks for several years. Only two other lines had more than one outbreak: Norwegian Cruise Line (4) and Princess Cruises (2). Three other lines — Carnival, Regent Seven Seas and American Canadian Caribbean Line — each had a single outbreak.

Holland America also led the industry in gastrointestinal illness events in 2007 with five outbreaks (tying with NCL) and in 2006 with seven outbreaks, according to the CDC data.

Notably, in 2008, there were a number of major lines, including Royal Caribbean, Celebrity, Disney and Cunard that did not have a single recorded outbreak in 2008.

So far in 2009 the CDC has recorded two gastro-intestinal outbreaks on ships, on the Celebrity Mercury (Jan. 3-17) and the Holland America Maasdam (Jan. 2-9).

Sometimes called the “24-hour flu,” norovirus is the most common cause of stomach illness in the United States, accounting for around half of all cases, according to the CDC. It breaks out regularly in schools, nursing homes, hospitals, offices and other places people congregate.