A unique strain of swine flu is the suspected killer of dozens of people in Mexico, where authorities closed schools, museums, libraries and theaters in the capital on Friday to try to contain an outbreak that has spurred concerns of a global flu epidemic.
The worrisome new virus -- which combines genetic material from pigs, birds and humans in a way researchers have not seen before -- also sickened at least eight people in Texas and California, though there have been no deaths in the U.S.
"We are very, very concerned," World Health Organization spokesman Thomas Abraham said. "We have what appears to be a novel virus and it has spread from human to human ... It's all hands on deck at the moment."
The outbreak caused alarm in Mexico, where more than 1,000 people have been sickened. Residents of the capital donned surgical masks and authorities ordered the most sweeping shutdown of public gathering places in a quarter century.
President Felipe Calderon said his government only learned late Thursday, with the help of international laboratories, what kind of virus Mexico is faced with. "We are doing everything necessary," he said after meeting with his Cabinet to coordinate a response. "We understand the seriousness of the problem."
The WHO was convening an expert panel to consider whether to raise the pandemic alert level or issue travel advisories.
It might already be too late to contain the outbreak, a prominent U.S. pandemic flu expert said late Friday.
Given how quickly flu can spread around the globe, if these are the first signs of a pandemic, then there are probably cases incubating around the world already, said Dr. Michael Osterholm at the University of Minnesota.
In Mexico City, "literally hundreds and thousands of travelers come in and out every day," Osterholm said. "You'd have to believe there's been more unrecognized transmission that's occurred."
There is no vaccine that specifically protects against swine flu, and it was unclear how much protection current human flu vaccines might offer. A "seed stock" genetically matched to the new swine flu virus has been created by the U.S. Centers for Disease Control, said Dr. Richard Besser, the agency's acting director. If the government decides vaccine production is necessary, manufacturers would need that stock to get started.
Authorities in Mexico urged people to avoid hospitals unless they had a medical emergency, since hospitals are centers of infection. They also said Mexicans should refrain from customary greetings such as shaking hands or kissing cheeks. At Mexico City's international airport, passengers were questioned to try to prevent anyone with flu symptoms from boarding airplanes and spreading the disease.
Epidemiologists are particularly concerned because the only fatalities so far were in young people and adults.
The eight U.S. victims recovered from symptoms that were like those of the regular flu, mostly fever, cough and sore throat, though some also experienced vomiting and diarrhea.
U.S. health officials announced an outbreak notice to travelers, urging caution and frequent handwashing, but stopping short of telling Americans to avoid Mexico.
Mexico's Health Secretary Jose Angel Cordoba said 68 people have died of flu and the new swine flu strain had been confirmed in 20 of those deaths. At least 1,004 people nationwide were sick from the suspected flu, he said.
The geographical spread of the outbreaks also concerned the WHO -- while 13 of the 20 deaths were in Mexico City, the rest were spread across Mexico -- four in central San Luis Potosi, two up near the U.S. border in Baja California, and one in southern Oaxaca state.
Scientists have long been concerned that a new flu virus could launch a worldwide pandemic of a killer disease. A new virus could evolve when different flu viruses infect a pig, a person or a bird, mingling their genetic material. The resulting hybrid could spread quickly because people would have no natural defenses against it.
Still, flu experts were concerned but not alarmed about the latest outbreak.
"We've seen swine influenza in humans over the past several years, and in most cases, it's come from direct pig contact. This seems to be different," said Dr. Arnold Monto, a flu expert with the University of Michigan.
"I think we need to be careful and not apprehensive, but certainly paying attention to new developments as they proceed."
The CDC says two flu drugs, Tamiflu and Relenza, seem effective against the new strain. Roche, the maker of Tamiflu, said the company is prepared to immediately deploy a stockpile of the drug if requested.
Both drugs must be taken early, within a few days of the onset of symptoms, to be most effective.
Cordoba said Mexico has enough Tamiflu to treat 1 million people, but the medicine will be strictly controlled and handed out only by doctors.
Mexico's government had maintained until late Thursday that there was nothing unusual about the flu cases, although this year's flu season had been worse and longer than past years.
The sudden turnaround by public health officials angered many Mexicans.
"They could have stopped it in time," said Araceli Cruz, 24, a university student who emerged from the subway wearing a surgical mask. "Now they've let it spread to other people."
The city was handing out free surgical masks to passengers on buses and the subway system, which carries 5 million people each day. Government workers were ordered to wear the masks, and authorities urged residents to stay home from work if they felt ill.
Closing schools across Mexico's capital of 20 million kept 6.1 million students home, as well as thousands of university students. All state and city-run cultural activities were suspended, including libraries, state-run theaters, and at least 14 museums. Private athletic clubs closed down and soccer leagues were considering canceling weekend games.
The closures were the first citywide shutdown of public gathering places since thousands died in the devastating 1985 earthquake.
Mexico's response brought to mind other major outbreaks, such as when SARS hit Asia. At its peak in 2003, Beijing shuttered schools, cinemas and restaurants, and thousands of people were quarantined at home.
In March 2008, Hong Kong ordered more than a half-million students to stay home for two weeks because of a flu outbreak. It was the first such closure in Hong Kong since the outbreak of SARS, or severe acute respiratory syndrome.
"It's great they are taking precautions," said Lillian Molina, a teacher at the Montessori's World preschool in Mexico City, who scrubbed down empty classrooms with Clorox, soap and Lysol between fielding calls from worried parents.
U.S. health officials said the outbreak is not yet a reason for alarm in the United States. The five people sickened in California and three in Texas have all recovered.
It's unclear how the eight, who became ill between late March and mid-April, contracted the virus because none were in contact with pigs, which is how people usually catch swine flu. And only a few were in contact with each other.
CDC officials described the virus as having a unique combination of gene segments not seen before in people or pigs. The bug contains human virus, avian virus from North America and pig viruses from North America, Europe and Asia. It may be completely new, or it may have been around for a while and was only detected now through improved testing and surveillance, CDC officials said.
The most notorious flu pandemic is thought to have killed at least 40 million people worldwide in 1918-19. Two other, less deadly flu pandemics struck in 1957 and 1968.