MEXICO CITY - New Zealand and Israel confirmed cases of swine flu on Tuesday, the latest countries hit by a new strain that has killed up to 149 people in Mexico and which threatens to become a pandemic.

The World Health Organization has raised its alert level to phase 4, indicating a significantly increased risk of pandemic.

Global markets slumped for a second day on Tuesday on fears the outbreak could snuff out fragile signs of economic recovery.

No one has died outside Mexico but more than 50 infected people have been found in the United States, six in Canada and two each across the Atlantic in Spain and Scotland. Possible cases were being tested in South Korea and Australia.

New Zealand said three of 11 people in a school group that visited Mexico had tested positive and it expected the others would also turn out to be positive when tests were completed.

Health Minister Tony Ryall said all those affected appeared to have only mild symptoms and had been responding to treatment.

The Israeli carrier, a 26-year-old man, had also recently returned from Mexico.

His condition is good but he is being kept hospitalized for observation, health ministry spokeswoman Einav Shimron said.

A honeymooning Scottish couple who recently returned from Cancun, one of Mexico's biggest beach resorts, were the first people in Britain to test positive for swine flu.

One of the mysteries of the current outbreak is why all cases outside Mexico have so far been relatively mild.

The WHO said the flu was being spread by human-to-human transmission but it did not advise any travel restrictions or border closures.

European and Asian stock markets retreated, with airline stocks taking another hit and drug makers posting gains. Investors cut their exposure to riskier currencies.

Oil dropped 2 percent, sinking below $50 a barrel.

Markets are doing what they tend to do, taking fright, said Howard Wheeldon, strategist at BGC Partners in London. But in my view, it's totally unnecessary.

Airlines braced for significant falls in traffic due to the outbreak.

Anything that shakes the confidence of passengers has a negative impact on the business. And the timing could not be worse given all of the other economic problems airlines are facing, international airlines lobby IATA said in a statement.

Britain, France, Germany and the United States issued travel alerts for Mexico, which relies on tourism as a main source of foreign currency. Japan advised its citizens in Mexico to consider returning home soon.

U.K. travel firms Thomson Holidays and First Choice said they had decided to repatriate their customers from Mexico and to cancel flights bound for Cancun on Tuesday. British Airways said it will continue to operate its services.

MEXICO HUNKERS DOWN

China promised to disclose any cases promptly. State-run newspapers urged officials to be open and avoid the kind of cover-up that brought panic during the SARS epidemic in 2003.

Asian companies stepped up precautions, restricting travel and advising staff on how to protect themselves.

Experts say that while it is impossible to stop the spread of the disease, efforts to slow its progress around the world could buy crucial time for countries to procure essential drugs.

The last flu pandemic, a Hong Kong flu outbreak in 1968, killed about one million people around the world.

In Mexico, epicenter of the latest outbreak, people from company directors to couriers wore face masks while airlines checked passengers for flu symptoms.

We will defeat this threat, Mexico City Mayor Marcelo Ebrard said 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.

Mexico City shut restaurants, bars, cinemas, stadiums 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. Some opted to work from home. Schools were closed until May 6.

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

Economic activity must continue, Labor Minister Javier Lozano told a news conference.

Danske Research said global markets were worried but not panicking. The outbreak could be used as an excuse to reduce risk after a period of increased appetite, it said. But if the WHO raised its alert level, it could hit harder.

The impact would obviously be biggest for countries involved and for emerging markets with weak medical infrastructure and lack of contingency plans for fighting pandemics, Danske said in a research note.

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 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. It was too early to identify the cause or geographical source of the virus.

The WHO said the first victims may not have known they were infected with a new type of flu requiring different treatment than normal, WHO spokesman Gregory Hartl said in Geneva.

They may not have received the medicines until late or may have been infected with other diseases reducing their immunity to the virus, he added.

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

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