More than 100 passengers on a luxurious 20-day Alaska cruise were sick with vomiting and diarrhea as a norovirus outbreak plagued the ship.A Princess Cruise ship docked in San Francisco on Thursday with 102 passengers and 23 crew members stricken with the harrowing virus, according to the US Centers for Disease Control and Prevention.Norovirus, more commonly referred to as the 'stomach flu,' is a highly contagious disease that causes gastrointestinal illness. It can be spread through direct contact, touching contaminated objects and eating food or drinking beverages that have come in contact with the virus. Infected individuals can still spread norovirus for two weeks or more after symptoms subside. The highly contagious virus is a nightmare outbreak for cruise ship passengers, who are often confined to the boat for days. The recent outbreak occurred on the cruise line's Ruby Princess ship and was first reported on June 28, according to the CDC. Just over three percent of passengers fell ill, while about two percent of crew members experienced symptoms. Princess Cruises experienced a norovirus outbreak on its recent voyage to Alaska, with over 100 passengers and 23 crew members infected Norovirus is a highly contagious illness that can spread for up to two weeks after symptoms subside The massive multi-week voyage departed San Francisco on June 12, traveled to Canada and Alaska, and returned to California this week. Princess Cruises reported that staff increased cleaning and disinfection procedures due to the outbreak and collected stool specimens from infected individuals for testing, the CDC said. Passengers and crew members who contracted the virus were isolated, and staff consulted with the CDC's Vessel Sanitation Program to contain the outbreak. 'Our crew responded promptly by implementing enhanced sanitation protocols across the ship, and cases have since decreased and remain low,' the cruise line said in a statement. Princess added that the ship was undergoing 'comprehensive cleaning and disinfection' to prepare for the next voyage. So far this year, the CDC has reported seven cruise ship outbreaks. Princess accounts for three of those cases. How to avoid norovirusThe CDC recommends the following steps to avoid contracting the gastrointestinal illnessWash your hands frequently Cook shellfish thoroughly and wash fruits and vegetablesClean and disinfect surfacesWash laundry in hot waterStay isolated for 2 days after symptoms stop The outbreak marks the third of the year on Princess Cruises, all of which were from norovirus Norovirus is spread through direct contact, touching contaminated objects and eating food or drinking beverages that have come in contact with the virus The cruise line was hit with two other norovirus cases on its Star Princess and Caribbean Princess ships during a voyage in mid-March and another one from April 28 to May 11. Lindblad Expeditions has also reported two norovirus outbreaks on its National Geographic Sea Bird ship this year. There have been two E.coli outbreaks this year as well. The first case was reported aboard Regent Seven Seas in January, and the second aboard Oceania Cruises in April. The Daily Mail has reached out to Princess Cruises for additional comment.