USA Today today says the U.S. Centers for Disease Control in Atlanta recorded just 13 outbreaks of gastrointestinal illness on cruise ships in 2009, down from 15 in 2008, 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, the last year for which data is available, 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 exceeds 2% of passengers and crew. When the number of cases exceeds 3% of passengers and crew the CDC issues a public report.