Fears of a global swine flu pandemic grew with new infections in the United States and Canada on Sunday, and millions of Mexicans stayed indoors to avoid a virus that has killed more than 100 people.
The death toll from the flu in Mexico has risen to 103 and about 400 people still are in hospitals around the country, Health Minister Jose Angel Cordova said on Sunday.
While the deaths have been limited to Mexico, the flu is spreading with 20 cases in the US and six in Canada, and possible cases as far afield as New Zealand, Europe and Israel.
There was a glimmer of hope, however, as the majority of the infected patients in Mexico have recovered from the illness, according to the government's figures.
The US declared a public health emergency and a top official at the Centres for Disease Control and Prevention said she feared there would be deaths in the US as the new strain of flu spreads.
The Mexican capital, one of the world's biggest cities, slowed to a snail's pace as millions of residents stayed at home, fearing infection. Many who ventured out wore masks and some companies asked employees to work from home on Monday.
Roman Catholics listened to mass on the radio rather than go to church, and baptisms and confirmations were canceled. Professional soccer games were played in empty stadiums, bars were closed and cyclists stayed off the road in the normally chaotic city of 20 million people. Public closures could last 10 days.
WHO fears pandemic
The World Health Organisation has declared the flu a "public health emergency of international concern" that could become a pandemic, or global outbreak of serious disease.
A pandemic would deal a major blow to a world economy already suffering its worst crisis in decades, and experts say it could cost trillions of dollars.
The 1968 "Hong Kong" flu pandemic killed about 1 million people globally.
The WHO warned the flu was changing fast and could mutate into a more dangerous form.
Officials in the US said they would release a quarter of the American stockpile of the antiviral drugs Tamiflu, made by Roche AG, and Relenza, from GlaxoSmithKline. Both have been shown to be effective against the new swine flu.
Flu is characterised 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.
Although it is called "swine flu" there is no evidence any of the cases stemmed from contact with pigs.
Asian countries ban pork
The Chinese government says it has banned hog and pork product imports from Mexico and parts of the US, with immediate effect.
The disease cannot be transmitted by eating properly cooked pork, but the Philippines has already brought in a similar ban.
No cases of swine flu have been been reported in China, which was at the centre of the Severe Acute Respiratory Syndrome (Sars) epidemic in 2003, but the government has urged citizens to immediately report any signs of flu.