Japan on high alert
A student who recently returned from North America has tested positive for H1N1 flu in Japan's fourth confirmed case of the virus, a government official said on Sunday.
Forty-eight confirmed in Mexico, two in the US, one each in Canada and Costa Rica
Countries with confirmed cases: Argentina, Australia, Brazil, Hong Kong, Mexico, the US, Canada, Colombia, Guatemala, Costa Rica, El Salvador, China, South Korea, New Zealand, Israel, Spain, Britain, Germany, Panama, Poland, Sweden, Austria, Switzerland, Denmark, France, Italy, Ireland, the Netherlands, Portugal, Japan, Norway
Annual influenza epidemics are thought to result in three to five million cases of severe illness and between 250,000 and 500,000 deaths around the world, WHO says
The Japanese teenager was among 49 passengers who were quarantined near Tokyo's Narita International Airport following confirmation of Japan's first three cases of the H1N1 influenza virus on Saturday.
Earlier, the health ministry said a teacher and two of the student's schoolmates, who had been on a high school trip to Canada, tested positive for the virus.
The other passengers, whose nationalities were not released by the authorities, arrived in Tokyo from the US city of Detroit on Friday on board the school party's flight.
"The four infected people have received medical treatment in hospital, but we have heard that they are not in extremely bad condition," Yutaka Ohno, a health ministry spokesman, said.
Japan has been on high alert as one of the year's busiest travel periods came to an end, the "Golden Week" of public holidays when tens of millions travel domestically and overseas.
Most people infected with the new strain, believed to be a hybrid of swine, bird and human influenza viruses, have either travelled to Mexico or been close to someone who has, but it is not the case with all of them.
Although the virus has been confirmed in 31 countries across the world, the vast majorities of cases have been in North America.
The US has overtaken its southern neighbour Mexico to become the country with the most number of cases.
It now has 2,254 confirmed cases in 44 of the 50 US states and the capital, Washington, with 104 people hospitalised; Mexico has recorded 1,626 cases including its 48 deaths, and done 5,580 tests followed by Canada with 224 cases.
The health authorities are now focusing on the characteristics of the strain and on developing a vaccine, said Anne Schuchat of the US Centres for Disease Control and Prevention (CDC).