TOKYO — An outbreak of the novel coronavirus on a cruise ship has prompted Japan to quarantine thousands of passengers and crew on board, with tests administered to hundreds on board. So far 174 people on the Diamond Princess have been diagnosed with the virus, but only a fraction of the 3,711 passengers and crew who arrived off Japan on Feb. 3 have been tested. Here are some questions and answers about how Japan is testing and treating those on board the vessel. How many people have been tested? Japan has tested nearly 500 people, gradually expanding from an initial group who displayed symptoms or had contact with a former passenger diagnosed with the virus after leaving the ship in Hong Kong last month. As new cases have emerged, authorities have tested those who had close contact with the most recently diagnosed passengers and crew. But so far the government has not announced plans to test everyone on the ship. How do you test for new coronavirus? Japan"s health authorities use a method called a real-time PCR (polymerase chain reaction) test. This offers preliminary detection of the virus mainly using biological samples taken from the nose or throat. This test is regarded as a reliable, fast way to detect viruses. But it takes several hours to produce results and can provide false negatives. The test works by amplifying and sequencing RNA which is matched against the genome of the novel coronavirus. But experts say it isn"t yet clear when in its lifecycle the virus becomes detectable. It may be hard to detect infections in people who don"t yet display symptoms, even though those people may be contagious. Why not test everyone on board? The Japanese government has faced questions about its decision to test small batches of passengers and crew, but says it is limited by existing capacity. At the moment it can only test up to 300 people a day. It says it hopes to be able to test 1,000 people per day by Feb. 18, the day before the ship"s quarantine ends. Health Minister Katsunobu Kato said the government was attempting to increase its testing capacity with help from universities and private businesses. "I want to administer PCR tests to everyone" on board the ship, Kato said Wednesday. And testing isn"t limited to those on board the ship. Japan has evacuated hundreds of citizens from Hubei, the Chinese province where the virus emerged. It tested the evacuees upon their arrival, and has carried out further tests for those displaying symptoms. In an illustration of the difficulties in detecting the virus, one man who arrived on an evacuation flight in late January tested negative twice for the virus before being diagnosed positive in a third test. Why is the ship in quarantine? Japanese authorities announced the ship would be quarantined after it emerged that a former passenger tested positive for the virus. Authorities say the quarantine is to protect people inside Japan but also to prevent the virus from spreading on the ship. Quarantine officers have required passengers to wear masks when they leave their rooms for brief stints on open decks. They have also been asked to stay a meter away from each other when outside. Crew are also taking precautions, wearing gloves and masks when delivering items to passengers. But the number of cases on board has continued to rise, though it is unclear whether the infections are new or just went undetected. When will the quarantine end? Japanese authorities have said the ship must stay in quarantine for 14 days, a period that will end on February 19 and is based on the generally accepted maximum incubation period for the virus. Passengers have expressed fears that newly detected cases could prompt authorities to effectively reset the 14-day clock, but so far there has been no indication of that. "The quarantine period of the #DiamondPrincess will come to an end on 19 February," the WHO said in a tweet on Monday. "The period will be extended beyond the 19 Feb as appropriate only for close contacts of newly confirmed cases. They need to remain in quarantine for 14 days from last contact with a confirmed case." — AFp