A student who recently returned from North America has tested positive for swine flu in Japan's fourth confirmed case of the virus, a government official said Sunday.
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 A(H1N1) influenza virus 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 authorities, arrived in Tokyo from the US city of Detroit 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," said Yutaka Ohno, a health ministry spokesman.
Six other students who were also quarantined and sent to hospital Saturday due to flu symptoms tested negative, the spokesman added.
Japanese authorities said the affected teacher and pupils, from the western city of Osaka, and their fellow passengers were taken to a hotel near the airport to be isolated for 10 days.
In a separate case, a woman in her 30s who had recently returned from the United States had been tested for swine flu in Kawasaki, near Tokyo, but was found not to have the disease, health ministry officials 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.