(AP) -- The schools and museums are closed. Sold-out games between Mexico's most popular soccer teams are being played in empty stadiums. Health workers are ordering sickly passengers off subways and buses. And while bars and nightclubs filled up as usual, even some teenagers were dancing with surgical masks on.
Across this overcrowded capital of 20 million people, Mexicans are reacting with fatalism and confusion, anger and mounting fear at the idea that their city may be ground zero for a global epidemic of a new kind of flu - a strange mix of human, pig and bird viruses that has epidemiologists deeply concerned.
Tests show 20 people in Mexico have died of the new swine flu strain, and that 48 other deaths were probably due to the same strain. The caseload of those sickened has grown to 1,004 nationwide, Mexico's Health Secretary Jose Angel Cordova said.
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.
Scientists have warned for years about the potential for a pandemic from viruses that mix genetic material from humans and animals. This outbreak is particularly worrisome because deaths have happened in at least four different regions of Mexico, and because the victims have not been vulnerable infants and elderly.
The most notorious flu pandemic, thought to have killed at least 40 million people worldwide in 1918-19, also first struck otherwise healthy young adults.
Authorities in the capital responded Friday with a sweeping shutdown of public places and events, urging people to stay home if they feel sick and to avoid shaking hands or kissing people on the cheeks.
Mexicans quickly got the message - and wanted to make sure their family members did, too.
Cristina Ceron, a 55-year-old waitress, called her daughter as soon as she got off work. "Please keep your mouth covered. And don't you eat street food," she pleaded through a white surgical mask.
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, along with a flurry of warnings from disease experts, left many angry and confused.
"Why did it break out, where did it break out? What's the magnitude of the problem?" said pizzeria owner David Vasquez, who was taking his family out to see "Monsters vs. Aliens" at a movie theater despite the urging of health officials that city residents stay home Friday night.
It was his son's 10th birthday, and he couldn't bear to cancel their outing. Vasquez said he would keep the family home the rest of the weekend.
The outbreak even hit Mexico's beloved national pastime - two sold-out football matches Sunday - Pumas vs. Chivas and America vs. Tecos - will be played in empty stadiums to prevent the spread of the disease.
Health workers also staffed the international airport and bus and subway stations, handing out masks and trying to steer away anyone who appeared sick. Many commuters wore masks, but there weren't enough to go around. One woman leaving a station nervously pulled her sweater over her face as her companion laughed and rolled his eyes.
A nearby pharmacy put up signs reading "We don't have masks" in black magic marker after selling out all 150 in stock.
Scientists have long been concerned that a new killer flu could evolve when different 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.
The WHO was convening an expert panel this weekend to consider whether to raise the pandemic alert level or issue travel advisories. The CDC and Canadian health officials were studying samples sent from Mexico, and some governments around Latin America said they would monitor passengers arriving on flights from Mexico.
But it may be too late to contain the outbreak, given how widespread the known cases are. If the confirmed deaths are the first signs of a pandemic, then cases are probably incubating around the world by now, said Dr. Michael Osterholm, a pandemic flu expert at the University of Minnesota.
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. Actually producing the vaccines could take months.
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.
Cordova said Mexico 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.
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.
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.
Miguel Cruz, a 20-year-old office supply store employee, said his mother sent him to ask about vaccines at a public hospital. He was given masks instead, which he and his girlfriend wore as they relaxed in a plaza.
A little girl in dirty clothes came over to sell them candy. They gave her mask, too.
"You know, they stay here and end up sleeping on the streets," said Cruz, watching the giggling girl scamper off.
In Mexico City's Zona Rosa neighborhood, teenagers with spiky hair and tight jeans laughed at the danger.
"People are giving too much importance to something that isn't that big of a deal," said Oscar Zarate, 19, shouting over the loud music and the jostling crowd outside a packed night club.
But his friend Leroy Villaluna was slipping a blue surgical mask on and off.
"Well, I guess I am a little afraid," Villaluna said with an embarrassed laugh. "And also, my mom was worried and told me that if I had to go out I should at least cover my mouth."
Associated Press Writers Traci Carl, Mark Stevenson, Carlos Rodriguez and Istra Pacheco in Mexico City; Mike Stobbe in Atlanta; Malcolm Ritter in New York; and Maria Cheng in London contributed to this report.
©2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Explore further: Liberia's Sirleaf sees signs of Ebola 'stabilisation'