Japanese authorities scrambled Saturday to track travelers who arrived on the same flight as three people diagnosed with the country's first confirmed cases of swine flu. Australia also joined the ranks of affected countries with its first confirmed case.

Authorities in Tokyo quarantined a high school teacher and two teenage students who returned Friday from a school trip to Canada after they tested positive at the airport.

In the Canadian province of Alberta, the chief medical officer on Friday confirmed the death of a woman infected with the virus. The woman, who was in her 30s and had other health problems, died April 28.

Officials said she had not left the country recently, but could not confirm whether she was in contact with anyone who had recently returned from Mexico, where swine flu has hit hardest. Dr. Andre Corriveau, Alberta's chief health officer, said 300 people who attended the woman's wake were being monitored for signs of the illness.

The World Health Organization has said based on past outbreaks, it is possible that a third of the world's population, or about 2 billion people, could become infected if this outbreak turns into a two-year pandemic. Independent experts agreed it was possible but pointed out that many would not show any symptoms.

People with chronic illnesses are at greatest risk for severe illness from the flu, along with the elderly and young children. So far, most of those with the swine flu in the U.S. and Mexico have been young adults.

A report by the U.S. Centers for Disease Control and Prevention said America's two swine flu deaths — a toddler and a pregnant woman who both died in Texas— each suffered from several other illnesses when they were infected with the virus.

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Asia has been largely spared from the virus that continues to claim lives in worst-hit Mexico, which announced Saturday that the number of deaths had climbed to 48 even as it emerged from a national shutdown that closed schools and businesses and shuttered churches and soccer stadiums.

But since the outbreak began last month, several countries, including Japan, have screened air travelers for flu symptoms. But news reports said as many as 11 people on the Northwest Airlines flight from Detroit that landed in Tokyo on Friday avoided those screenings. Japan's national laboratory confirmed the virus in the teacher and two students.

The ministry said at least 13 people — believed to be separate from the reported 11 that the ministry was still investigating — had gone on to other destinations in transit from that flight, and efforts were under way to contact them through the World Health Organization.

Japanese Health and Welfare Minister Yoichi Masuzoe acknowledged it would be difficult to trace all those who came into contact with the three infected Japanese, who visited Ontario on a home-stay program with about 30 other students. The three were isolated and recovering at a hospital near Narita International Airport.

"There are limitations to what we can do, but we will continue to monitor the situation and strengthen or relax such measures as needed," he told reporters.

Public broadcaster NHK TV urged people who were aboard the flight to call a special telephone number for consultations. So far, 49 have been traced and will be monitored for 10 days, officials said.

But a handful of cases have cropped up in the region, including in South Korea and Hong Kong. The Chinese territory quarantined more than 200 people in a hotel after confirming its first case in a guest a week ago. They were released on Friday, and many were unable to contain their delight as they poured from the building for the first time in seven days. One man hugged a police officer and broke into song.

Australia reported its first case on Saturday in a woman it said was no longer infectious. She first noticed her symptoms while traveling in the U.S., federal Health Minister Nicola Roxon told reporters.

New Zealand — the first country in the Asia-Pacific region to confirm cases — reported two more on Saturday for a total of seven. The two high school students returned last month from a school trip to Mexico. Six of the country's cases were in students and a teacher on that trip; the seventh traveled on the same plane as the group.