Efforts by cruise lines to sanitize ships appear to be working, as passengers on boats once stricken with outbreaks of stomach ailments are reporting fewer illnesses, federal officials reported today.

More than 1,500 cruise ship passengers became sick in November and December, prompting the Centers for Disease Control and Prevention to investigate several lines. Holland America's Amsterdam, Disney Cruise Line's Magic and Carnival Corp.'s Fascination cleaned their ships after outbreaks and only a handful of illnesses have been reported since.

"These numbers reflect all the work they did in cleaning and sanitizing the ship," said Dave Forney of the CDC on Holland America's efforts to clean the Amsterdam. Nearly 1,000 people fell ill on four recent trips by the ship.

Last month, 275 people became ill aboard the Magic and 194 people fell sick on the Fascination.

The Norwalk and Norwalk-like viruses -- which cause diarrhea, stomach pain and vomiting for up to 48 hours -- are spread by close contact with infected people or the things they have touched. The CDC said the ships' food and water systems do not appear to be at fault.

The CDC stopped short of saying the recent rash of cruise ship illnesses was over.

The CDC still is monitoring another cruise ship, P&O UK's Oceana, which reported 293 ill passengers and crew as of today. The CDC is analyzing samples from that ship's outbreak.