This item was written by Fran Golden, a cruise expert and Trip Coach for Budget Travel magazine. Golden is serving as Guest Editor of The Cruise Log while USA TODAY Cruise Editor Gene Sloan is away.
The extensive cleaning delayed the departure of the ship by more than five hours on Wednesday.
A spokeswoman said it was too soon to tell whether the illness had been contained; whether there were any sick passengers or crew on the ship's current sailing.
A total of 53 (or 2.4%) of 2,196 passengers were affected by the gastrointestinal illness during the previous sailing, a 10-day cruise that had departed San Francisco on June 19 and returned on Wednesday.
Norovirus is a common illness, easily transmitted from person-to-person. Cruise lines that call at U.S. ports are required to report cases to the U.S. Centers for Disease Control and Prevention (CDC), with a separate notification if more than 2% of passengers and crew are ill. The CDC issues a public report if more than 3% of those onboard are affected.
The Sea Princess' current norovirus issues began in May when 44 passengers were reported ill on the ship's May 10 sailing from San Francisco to Alaska.
Two larger outbreaks followed: On the May 20 cruise, 128 passengers and 13 crew fell ill; and on the May 30 sailing, 135 passengers and six crew were ill.
The cruise line's efforts to stem the bug included an extensive disinfection after passengers on the May 30 cruise disembarked and before new passengers were allowed to board on June 9.
The efforts seemed to work.
"We did experience an increased number of passengers with norovirus on three earlier Sea Princess cruises, but due to extra sanitation efforts and diligent communication with our passengers, no heightened occurrence of illness occurred on the June 9 cruise," Princess said in a statement. "We believe the reintroduction of the virus on the current cruise was inadvertently brought on board by embarking passengers, as we know norovirus is currently circulating widely throughout North America."
Princess said at the first sign of illness on the June 19 cruise, the line "initiated additional enhanced sanitation procedures to interrupt the person-to-person spread of this virus."
The cruise line said it followed CDC procedures including isolating ill passengers in cabins "until they were non-contagious."