With 24 new suspected cases of the swine flu reported Saturday in Mexico City alone, schools were ordered closed and all public events suspended until further notice - including more than 500 concerts and other gatherings in the metropolis of 20 million.
A hot line fielded 2,366 calls in its first hours from frightened city residents who suspected they might have the disease. Soldiers and health workers handed out masks at subway stops, and hospitals dealt with crowds of people seeking help.
The World Health Organization's director-general, Margaret Chan, said the outbreak of the never-before-seen virus is a very serious situation and 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 a telephone news conference in Geneva. "A new disease is by definition poorly understood."
This virus is a mix of human, pig and bird strains that prompted WHO to meet Saturday to consider declaring an international public health emergency - a step that could lead to travel advisories, trade restrictions and border closures.
But experts at the WHO and the U.S. Centers for Disease Control say the nature of this outbreak may make containment impossible. Already, more than 1,000 people have been infected in as many as 14 of Mexico's 32 states, according to daily newspaper El Universal. Tests show 20 people have died of the swine flu, and 48 other deaths were probably due to the same strain.
The CDC and Canadian health officials were studying samples sent from Mexico, and airports around the world were screening passengers from Mexico for symptoms of the new flu strain, saying they may quarantine passengers.
But CDC officials dismissed the idea of trying that in the United States, and some expert said it's too late to try to contain spread of the virus.
They noted there had been no direct contact between the cases in the San Diego and San Antonio areas, suggesting the virus had already spread from one geographic area through other undiagnosed people.
"Anything that would be about containing it right now would purely be a political move," said Michael Osterholm, a University of Minnesota pandemic expert.
Mexican President Felipe Calderon said his government only discovered the nature of the virus late Thursday, with the help of international laboratories. "We are doing everything necessary," he said in a brief statement.
But the government had said for days that its growing flu caseload was nothing unusual, so the sudden turnaround angered many who wonder if Mexico missed an opportunity to contain the outbreak.
"Why did it break out, where did it break out? What's the magnitude of the problem?" pizzeria owner David Vasquez said while taking his family to a movie Friday night, despite warnings to stay out of theaters.
Across Mexico's capital, residents reacted with fatalism and confusion, anger and mounting fear at the idea that their city may be ground zero for a global epidemic.
Authorities urged people to stay home if they feel sick and to avoid shaking hands or kissing people on the cheeks.
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.
Jose Donasiano Rosales, 69, got nervous on the subway and decided to get out one stop early.
"I felt I couldn't be there for even one more station," Donasiano said as he set up a rack to sell newspapers on a busy thoroughfare. "We're in danger of contagion. ... I'm worried."
The same virus also sickened at least eight people in Texas and California, though there have been no deaths north of the border, puzzling experts at the U.S. Centers for Disease Control and Prevention.
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. No vaccine specifically protects against swine flu, and it is 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 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.
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.
Mexico's Health Secretary Jose Angel Cordova said the country has enough Tamiflu to treat 1 million people - only one in 20 people in greater Mexico City alone - and that the medicine will be strictly controlled and handed out only by doctors.
Scientists have warned for years about the potential for a pandemic from viruses that mix genetic material from humans and animals. Another reason to worry about this virus is that the dead so far don't include vulnerable infants and elderly. The Spanish flu pandemic, thought to have killed at least 40 million people worldwide in 1918-19, also first struck otherwise healthy young adults.
At Mexico's National Institute of Respiratory Illnesses, Adrian Anda waited to hear whether his 15-year-old daughter had the frightening new disease. She had been suffering a cough and fever for a week.
"If they say that it is, then we'll suffer. Until then, we don't want to think about it," he said.
Associated Press Writers David Koop and Peter Orsi 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.