The outbreak in Japan went from just four cases over the weekend to 158 as infections were confirmed in the port city of Kobe and nearby Osaka, which is Japan's second-biggest urban area. The new wave of infections did not have a clear connection to foreign travel, as the initial one did, and involved primarily teenagers.
None of the confirmed patients were in serious condition, a health ministry official said.
Health and Welfare Minister Yoichi Masuzoe indicated that airport quarantine efforts would be scaled down and urged flexibility in anti-flu measures since the swine flu is now believed to be no more infectious than seasonal influenza. Health officials want to make sure that hospital beds are not clogged up with mild patients and that business and community activities are paralysed due to overreaction.
However, the government said it would do its best to quell the spread of the disease, and keep schools shut at least through the end of the week. Companies discouraged unnecessary business trips.
Japan had worked hard to keep the new flu strain from spreading within its island borders, establishing testing centres at airports and encouraging people in high-risk jobs to wear masks and wash their hands carefully.
But those defences appeared to have been breached. On Saturday, officials reported the first case of an infection occurring inside the country - not being brought in from abroad - and it remained unclear where the bulk of the newly infected people came into contact with the virus.
Reports said dozens of the students had played in school volleyball games, but officials could not confirm if that had any relation to their infections.
Experts also suspect that a domestic outbreak may have been triggered silently weeks before Japan caught its first cases at its main airport in early May.
The new wave would make Japan the fourth-most infected country in the world, after Mexico, the United States and Canada, according to several Japanese media reports. Officials admitted the new figures were disturbing. -- AP