Governments around the world have been hurrying to contain the spread of a new swine flu virus after outbreaks were reported in Mexico, the US and Canada.
At least 100 people are now suspected to have died of the disease in Mexico.
The UN has warned the virus has the potential to become a pandemic, but said the world was better prepared than ever to deal with the threat.
Stocks of anti-viral medicines are being readied and travellers are being screened at some airports for symptoms.
Mexican Health Secretary Jose Angel Cordova said suspected swine flu cases in his country had risen to 1,614 including 103 deaths.
Of those, 20 deaths are confirmed to have been caused by the new virus.
The US, where 20 people are confirmed to have caught the virus, has declared a public health emergency.
There are also confirmed cases in Canada, and investigations are being carried out on suspected cases in Spain, Israel and New Zealand.
In most cases outside Mexico, people have been only mildly ill and have made a full recovery.
The World Health Organization (WHO), the UN’s health agency, has said the swine flu virus could be capable of mutating into a more dangerous strain.
But officials say they need more information on the virus before deciding whether to raise the global pandemic alert phase.
The WHO is advising all countries to be vigilant for seasonally unusual flu or pneumonia-like symptoms among their populations – particularly among young healthy adults, a characteristic of past pandemics.
Only a handful of the Mexican cases have so far been laboratory-confirmed as swine flu, while in the US confirmed cases had only mild symptoms.
Health experts want to know why some people become so seriously ill, while others just develop a cold, the BBC’s Imogen Foulkes reports from Switzerland.
Dr Keiji Fukuda, WHO’s assistant director-general in charge of health security, said all countries were “looking at the situation seriously” but that a true picture of the extent of the virus was still emerging.
H1N1 is the same strain that causes seasonal flu outbreaks in humans but the newly detected version contains genetic material from versions of flu which usually affect pigs and birds.
It is spread mainly through coughs and sneezes.
Officials said most of those killed so far in Mexico were young adults – rather than more vulnerable children and the elderly.
There is currently no vaccine for the new strain of flu but severe cases can be treated with antiviral medication.
Dr Fukuda said years of preparing for bird flu had boosted world stocks of antivirals.
It is unclear how effective currently available flu vaccines would be at offering protection against the new strain, as it is genetically distinct from other flu strains.
WHO experts will meet again in Geneva on Tuesday to discuss whether to raise the pandemic alert phase.
In the US, eight cases have been confirmed among New York students, seven in California, two in Texas, two in Kansas and one in Ohio.
“I do fear that we will have deaths,” Dr Anne Schuchat of the US Centers for Disease Control and Prevention (CDC) told reporters.
The Canadian cases were recorded at opposite ends of the country: two in British Columbia in the west, and four in the Atlantic province of Nova Scotia.
Several countries in Asia and Latin America have begun screening airport passengers for symptoms.
Suspected cases have been detected beyond Mexico, the US and Canada:
Ã¢â‚¬Â¢ In New Zealand, two school groups that recently visited
Mexico have reported illnesses – ten students from one school tested positive for Influenza A, making it “likely” they are infected with swine flu, and three in the other school were being tested
Ã¢â‚¬Â¢ France and Spain have both reported cases of people becoming ill after returning from Mexico and are carrying out tests
Ã¢â‚¬Â¢ In Israel, medics are testing a 26-year-old man who has been taken to hospital with flu-like symptoms after returning from a trip to Mexico
Ã¢â‚¬Â¢ Two people in Queensland, Australia, are being tested after developing flu-like symptoms on returning from Mexico
Ã¢â‚¬Â¢ The Brazilian authorities say one man was taken into hospital as a precaution after he became ill following a visit to Mexico
The BBC’s Ros Atkins dons his face mask to explore Mexico City
With Mexico City apparently the centre of infection, many people are choosing to leave the city, the BBC’s Stephen Gibbs reports.
Schools, universities and even most bars and restaurants will remain closed for several days and though Sunday church services went ahead, priests were asked to place Communion wafers in people’s hands rather than on their tongues.
Some people are beginning to worry about the effects swine flu is having on their livelihoods and the Mexican economy in general, our correspondent says.
The World Bank is providing Mexico with more than $200m in loans to help it deal with the outbreak.
Fear of the virus is expected to lead to many tourists cancelling their holidays and Mexican exports are already beginning to be affected.
Russia has banned imports of raw pork and pork products from Mexico and the US states of California, Texas and Kansas until further notice as a precaution.
Dr Fukuda said on Sunday there was no proof that eating pork would lead to infection.
“Right now we have no evidence to suggest that people are getting exposed, or getting infected, from exposure to pork or to pigs, and so right now we have zero evidence to suspect that exposure to meat leads to infections,” he said.