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.
