(AP) -- As Mexico struggled against the odds Saturday to contain a strange new flu that has killed 68 and perhaps sickened more than 1,000, it was becoming clearer that the government hasn't moved quickly enough to head off what the World Health Organization said has the potential to become a global epidemic.
The World Health Organization said the outbreak has become a "public health emergency of international concern" and asked countries around the world to step up reporting and surveillance of the disease and implement a coordinated response to contain it.
But Mexicans were dying for weeks at least before U.S. scientists identified the strain - a combination of swine, bird and human influenza that people may have no natural immunity to. Now, even controlling passengers at airports and bus stations may not keep it from spreading, epidemiologists say.
The disease has already reached Texas, California and Kansas, and 24 new suspected cases were reported Saturday in Mexico City alone, where authorities suspended schools and all public events until further notice. More than 500 concerts, sporting events and other gatherings were canceled in the metropolis of 20 million.
The Mexican government issued a decree authorizing President Felipe Calderon to invoke special powers letting the Health Department isolate patients and inspect homes, incoming travelers and baggage.
Officials said the decree gives clear legal authority to Health Department workers who might otherwise face reprisals.
Health workers and soldiers joined a broad effort at airports and bus stations to keep people with disease from traveling though or out of the city. But with confirmed swine flu cases in at least 6 states - and possibly as many as 14 - the efforts seemed unlikely to stop the spread of the disease.
At Mexico City's international airport, health workers passed out written questionnaires seeking to identify passengers with flu symptoms. Surgical masks and brochures were handed out at bus and subway stations. The U.S. embassy in Mexico posted a message advising U.S. citizens to avoid large crowds, shaking hands, greeting people with a kiss or using the subway.
Particularly difficult in a metropolis as crowded as Mexico City was the embassy's advice to maintain "a distance of at least six feet from other persons may decrease the risk of exposure."
Early detection and treatment are key to stopping any outbreak. WHO guidance calls for isolating the sick and blanketing everyone around them with antiviral drugs such as Tamiflu.
Now, with patients showing up all across Mexico and its teeming capital, simple math suggests that kind of response is impossible.
Mexico appears to have lost valuable days or weeks in detecting the new virus.
Health authorities started noticing a threefold spike in flu cases in late March and early April, but they thought it was a late rebound in the December-February flu season.
Testing at domestic labs did not alert doctors here to the new strain, although U.S. authorities detected an outbreak in California and Texas last week.
Perhaps spurred by the U.S. discoveries, Mexico sent 14 mucous samples to the CDC April 18 and dispatched health teams to hospitals looking for patients with severe flu or pnuemonia-like symptoms.
Those teams noticed something strange: The flu was killing people aged 20 to 40. Flu victims are usually either infants or the elderly.
As recently as Wednesday, authorities were referring to it as a late-season flu.
But mid-afternoon Thursday, Mexico City Health Secretary Dr. Armando Ahued said, officials got a call "from the United States and Canada, the most important laboratories in the field, telling us this was a new virus."
"That was what led us to realize it wasn't a seasonal virus ... and take more serious preventative measures," Health Secretary Jose Cordova said.
Some Mexicans suspected the government had been less than forthcoming. "They always make a big deal about good things that happen, but they really try to hide anything bad," Mexico City paralegal Gilberto Martinez said.
Soldiers and health workers handed out masks at subway stops, and hospitals dealt with crowds of people seeking help. A hot line fielded 2,366 calls in its first hours from frightened city residents who suspected they might have the disease.
Doctors reported that anti-viral medications and even steroids were working well against the disease, noting no new deaths had been reported in the capital in the last day.
Airports around the world were screening travelers from Mexico for flu symptoms. But containing the disease may not be an option, because more than 1,000 people have been infected in as many as 14 of Mexico's 32 states, according to daily newspaper El Universal.
"Anything that would be about containing it right now would purely be a political move," said Michael Osterholm, a University of Minnesota pandemic expert.
Ahued, the capital's health secretary, said Mexico City may not even be the epicenter of the outbreak.
"The country's best health care facilities are concentrated in the city," he said. "All the cases here get reported, that's why the number is so high."
World Health Organization Director-General Margaret Chan said the outbreak of the never-before-seen virus has "pandemic potential." But she said it is still too early to tell if it would become a pandemic.
"The situation is evolving quickly," Chan said in Geneva. "A new disease is by definition poorly understood."
This virus is a mix of human, pig and bird strains that can be transmitted from human-to-human.
Scientists have warned for years about the potential for a pandemic from viruses that mix genetic material from humans and animals. The WHO held off on raising its pandemic alert level, citing the need for more information.
Another reason to worry is that authorities said the dead so far don't include vulnerable infants and elderly. The Spanish flu pandemic, which killed at least 40 million people worldwide in 1918-19, also first struck otherwise healthy young adults.
This swine flu and regular flu can have similar symptoms - mostly fever, cough and sore throat, though some of the U.S. victims who recovered also experienced vomiting and diarrhea. But unlike with regular flu, humans don't have natural immunity to a virus that includes animal genes - and new vaccines can take months to bring into use.
The same virus also sickened at least 10 people in the United States, though there have been no deaths north of the border.
A "seed stock" genetically matched to the new swine flu virus has been created by the CDC, 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.
None of that provided any easy answers to Mexico City residents, who reacted with fatalism and confusion, anger and mounting fear at the idea that their city may be ground zero for a global epidemic.
Outside Hospital Obregon in the capital's middle-class Roma district, a tired Dr. Roberto Ortiz, 59, leaned against an ambulance and sipped coffee Saturday on a break from an unusually busy shift.
"The people are scared," Ortiz said. "A person gets some flu symptoms or a child gets a fever and they think it is this swine flu and rush to the hospital."
He said none of the cases so far at the hospital had turned out to be swine flu.
Associated Press Writers David Koop in Mexico City; Frank Jordans in Geneva; Mike Stobbe in Atlanta; Malcolm Ritter in New York; and Maria Cheng in London contributed to this report.
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