The world's top epidemiologists are fighting to curb the spread of a new strain of flu that has killed up to 149 people in Mexico in a couple of weeks and may have spread as far as Asia.

Mexicans from company directors to delivery men on tricycles wore face masks while airlines checked passengers for flu symptoms and governments warned against non-essential travel to Mexico. Health experts assessed the risk of the virus becoming the first flu pandemic in more than 40 years.

The fast-spreading bug is curable if treated quickly with antiviral medicine but no one is naturally immune to it and the World Health Organisation has lifted its pandemic alert to phase 4, indicating the risk of a deadly global outbreak.

No one has died from the swine flu outside Mexico but it has infected more than 50 people in the US, six in Canada and three across the Atlantic in Spain and Scotland.

Possible cases were being tested as far away as Norway and South Korea.

'We will defeat this threat,' Mexico City Mayor Marcelo Ebrard vowed late on Monday as several hundred people suspected to be suffering from the flu were treated in hospitals and life in the normally hectic capital took on an eerie hush.

The last pandemic, a Hong Kong flu outbreak in 1968, killed about 1 million people.

Mexico City shut restaurants, bars, cinemas, stadiums, gyms and some government offices to stop the infection from spreading.

Unsure how worried they should be, people stocked up on food, drinking water, rental movies and surgical masks and canceled hair appointments. Some opted to work from home and schools were closed across the country until May 6.

Facing a dent to tourism and trade -- key motors of an economy that is already tipping into recession with the global downturn -- Mexico said it would not order a mass closure of businesses to try to contain the infection.

“Economic activity must continue,' Labour Minister Javier Lozano told a news conference on Monday evening.

Markets rattled

Worldwide, seasonal flu kills between 250,000 and 500,000 people in an average year. The new strain is worrying as it spreads rapidly between humans and there is no vaccine for it.

Most of the fatalities have been people aged between 20 and 50, an ominous sign because a hallmark of past pandemics has been the high rate of fatalities among young adults.

Mexican media have speculated the flu may have originated at a pig farm in the tropical southeastern state of Veracruz.

But Health Minister Jose Angel Cordova said the first case that alerted authorities to a possible rogue flu strain was in the southern state of Oaxaca. He said it was too early to identify the cause or geographical source of the virus.

Officials say the virus is not caught from eating pig meat products but several countries banned US pork imports.

As the death toll in Mexico rose to up to 149 and Cordova warned the figure would keep rising, UK, France, Germany and the US issued travel alerts for Mexico, which relies on tourism as its No. 3 source of foreign currency.

The heavy volume of visitors only underscores the risk of contagion. Airlines fly more than 1 million passenger seats in and out of Mexico's international airport each week.

The flu crisis, which blew up over the weekend, rocked financial markets on Monday, hitting oil prices, the dollar, Mexico's peso and airline stocks, although shares in some drug manufacturers gained.

Uncertainty over how bad the outbreak could get soured recent optimism that trillions of dollars injected into the world economy might soon start to slow the global downturn.

In Mexico, analysts say the flu crisis could shave half a percentage point off Mexico's growth this year as tourists stay away and Mexicans spend days at home with their purses shut. – Reuters