Reuters - Fears of a global swine flu pandemic grew with new infections in the United States and Canada on Sunday, and millions of Mexicans hid indoors to avoid a virus that has already killed up to 81 people.
While the only deaths so far have been in Mexico, the flu is spreading with 20 cases in the United States and four in Canada, and possible infections also popped up as far afield as Europe and New Zealand.
New York Mayor Michael Bloomberg confirmed on Sunday that eight schoolchildren there had caught the swine flu virus, although the cases were mild and the illness did not appear to be spreading rapidly to the general population.
Another 12 cases have been confirmed in California, Kansas, Texas and Ohio, and Homeland Security Secretary Janet Napolitano said the government would declare a public health emergency.
Canadian health authorities said four cases were confirmed there. In New Zealand, 10 students from a school party that had been in Mexico were being tested after showing flu-like symptoms.
The World Health Organization has declared the flu, of a type never seen before, a "public health emergency of international concern" and says it could become a pandemic, or a global outbreak of serious disease.
A pandemic would deal a major blow to a world economy already suffering its worst recession in decades, and experts say it could cost trillions of dollars.
A 1968 "Hong Kong" flu pandemic killed about 1 million people globally.
Mexico City, one of the world's biggest cities, practically ground to a halt on Sunday with restaurants, cinemas and churches closing their doors and millions staying at home as officials tested 1,300 suspect cases.
Worshipers were told to follow Sunday church services on television and some residents abandoned the capital, a rambling, chaotic city of some 20 million people.
Michelle Geronis, 22, a film student, took a bus to be with her family in the central state of Aguascalientes.
"My parents heard the news and said, 'You know what? You'd better get here,'" she said.
The U.S. Centers for Disease Control and Prevention advised Americans to see a doctor if they had good reason to suspect swine flu. "If you do not have symptoms you should not get tested," the CDC's acting director Dr. Richard Besser told a White House briefing.
U.S. officials also said they would release a quarter of the country's stockpile of the antiviral drugs Tamiflu, made by Roche AG, and Relenza, made by GlaxoSmithKline. Both have been shown to be effective against the new swine flu.
Flu is characterized by a sudden fever, muscle aches, sore throat and dry cough. Victims of the new strain have also suffered more vomiting and diarrhea than is usual with flu.
White House spokesman Robert Gibbs said the U.S. government is following developments on the swine flu closely and would update the public regularly.
In Spain, doctors checked three people who had returned from visiting Mexico and reported flu-like symptoms.
The new flu strain, a mixture of four different swine, bird and human viruses, poses the biggest risk of a large-scale pandemic since avian flu surfaced in 1997, killing several hundred people.
New flu strains can spread quickly because no one has natural immunity to them and a vaccine takes months to develop.
WHO Director-General Dr. Margaret Chan urged greater worldwide surveillance for any unusual outbreaks of influenza-like illness.
The outbreak has snowballed into a huge headache for Mexico, already grappling with a violent drug war and economic
slowdown, and has become one of the biggest global health scares in years.
"(We are) monitoring minute by minute the evolution of this problem across the whole country," Mexican President Felipe Calderon said as health officials counted suspected infections from the tropical south to the arid northern border.
Authorities across Asia, who have had to grapple with deadly viruses like H5N1 bird flu and SARS in recent years,
snapped into action. At airports and other border checkpoints in Hong Kong, Malaysia, South Korea and Japan, officials screened travelers for any flu-like symptoms.