A luxury cruise liner quarantined off Yokohama has the biggest cluster of coronavirus cases outside China, with nearly 5 per cent of its passengers and crew infected.
There were 174 cases on the Diamond Princess as of yesterday, out of 3,711 people on the ship manifest.
A Health Ministry quarantine officer who had worked on the ship last week - and taken protective measures in line with government protocol - has also tested positive.
All these cases can be traced back to one Hong Kong man who was on the ship from Jan 20 to 25.
Infection clusters of the new coronavirus, which originated in the central Chinese city of Wuhan, have emerged worldwide - from a department store in the eastern Chinese city of Tianjin to a Hong Kong apartment block to a ski chalet in the French Alps.
"In general, if people live or work in close, cramped quarters where there is little natural ventilation or air exchange, the risk of transmission of respiratory pathogens is very high," Associate Professor Hsu Li Yang, programme leader of infectious diseases at the National University of Singapore's Saw Swee Hock School of Public Health, told The Straits Times.
Many unknowns remain over the virus, which may potentially complicate the best efforts by governments to contain its spread. There have been conflicting expert accounts of how virulent the strain is, how long the incubation period is and how infectious an asymptomatic patient could be.
There is also still no consensus on the virus' mode of transmission, including if it is airborne and can be spread via central ventilation or air-conditioning systems.
In Wuhan, which is currently under lockdown, a sports stadium and an exhibition hall have been converted into makeshift isolation shelters for sick patients.
But with the healthcare system under pressure, experts are concerned that some of the patients may not have been properly screened and could in fact be suffering from something else.
In Hong Kong, two residents who live 10 storeys apart in the same apartment block tested positive for the coronavirus, with the authorities blaming the block's piping system.
Number of cases on cruise liner Diamond Princess, now quarantined off Yokohama in Japan. A quarantine officer who had worked on the ship also tested positive. A total of 3,711 people were listed on the ship manifest.
Number of patients in China's Tianjin city who worked or shopped at a department store in Baodi district, or had close contact with staff or customers. About 11,700 people were put in quarantine.
Number of cases linked to a ski resort in the French alpine town of Les Contamines-Montjoie. Five Britons and one man from Spain had stayed at the same resort as Briton Steve Walsh, who attended a conference in Singapore last month. He was tested positive after returning to Britain, where he infected five more people, including healthcare workers.
Number of cases linked to a residential building in Hong Kong's Tsing Yi district, New Territories. About 200 residents were placed under quarantine.
Source: REUTERS, NYTIMES
More than 200 residents of the 35-storey building were evacuated overnight, with memories of the 2003 severe acute respiratory syndrome (Sars) outbreak fresh in the minds of many. Another three confirmed cases have since been linked to the building.
During the Sars outbreak, 329 residents of a crowded housing complex were infected through the drainage system, killing 42.
Over in Tianjin, which has 102 cases, at least 33 have been traced to a department store in the district of Baodi. About 11,700 shoppers and employees who were at the mall late last month have been quarantined.
Mr Steve Walsh, a Briton who contracted the virus at a business meeting at the Grand Hyatt in Singapore, subsequently passed the virus on to at least 11 others - among them five fellow skiers at a French alpine resort despite him being asymptomatic at the time.
Still, nowhere outside China has the spread been as pronounced as on board the Diamond Princess. The ship has been under quarantine since Feb 5, with passengers ordered to stay in their rooms, although crew members continue to live in shared quarters.
"Quarantine is meant to prevent the spread of infectious diseases to a population," Prof Hsu said. "They are effective in protecting that population in question and most, if not all, countries have laws that permit quarantine."
Japan, which currently has the capacity to test only up to 300 cases a day, is wrestling with the logistical and cost feasibility of screening every person on board the ship.
The 174 who tested positive are among only 492 people who have been screened. Prime Minister Shinzo Abe said yesterday that the government is working to raise this capacity to more than 1,000 cases a day by next Tuesday.
Japan is following the lead of the United States' Centres for Disease Control and Prevention, whose principal deputy director Anne Schuchat said in a letter to passengers that "remaining in your room on the ship is the safest option to minimise your risk of infection".
"Alternatives might increase the risk of spread of infection," she said, adding that while much is unclear about how the virus is transmitted, there is "no current evidence to suggest that the virus spreads between rooms on a ship through the air-handling system".
For its part, Princess Cruises, the operator of the quarantined ship, said: "The heating ventilation air-conditioning filtration system on our ships is comparable to those used by land-based hotels, resorts and casinos."