MEXICO CITY AND LOS ANGELES — An outbreak of swine flu that may have killed as many as 60 people prompted authorities in Mexico City to close schools Friday throughout the sprawling city of 20 million and order emergency health measures in an attempt to contain the disease.
In the United States, officials said they had found one new case in San Diego, bringing the number of U.S. cases to eight. All have recovered fully. The World Health Organization in Geneva said the strain in Mexico was identical to the one that has been detected in California and Texas.
Nervous parents in Mexico City formed long lines at clinics Friday morning, some wearing surgical masks and carrying toddlers. They were full of questions, about symptoms, how they could stay home from work to care for the sick, where to get the medicines.
"We are monitoring the evolution of the epidemic and, so far, it is under control," national Health Secretary Jose Angel Cordova said. He said the rate of deaths was slowing and that there were no plans to close the country's borders because of the outbreak.
But it was possible that schools would remain closed next week, officials said, adding that they were considering whether to shut down businesses and offices.
In Mexico state, museums, theaters and cinemas will be closed for the weekend, and Mexico City Mayor Marcelo Ebrard canceled public events, including concerts and sports matches.
Cordova said 20 deaths have been confirmed as being caused by swine flu and 40 other cases are still being investigated. He said 1,004 people are reported to be ill with flu symptoms, which include a high fever, severe headache and persistent cough.
International health officials said they were considering whether to raise the alert for a possible pandemic -- a global outbreak.
"We have what appears to be a novel virus and it has spread from human to human," Thomas Abraham, a spokesman for WHO, said in Geneva, news agencies reported. Human-to-human transmission is the crucial requirement for a new virus to precipitate a large-scale outbreak.
But researchers at the U.S. Centers for Disease Control and Prevention have found nothing connecting the U.S. victims or any common behaviors, acting director Dr. Richard Besser said Friday in a telephone news conference. That suggests "there has been transmission through several cycles" -- meaning that several intermediaries passed it among themselves before the virus reached the identified victims.
If that is the case, Besser said, many people have been exposed to the virus and it is too late to contain a potential outbreak in the United States. The good news is that none of the intermediaries appear to have become seriously ill, suggesting that the disease is not especially virulent.
None of the U.S. victims has had any contact with pigs and only one has traveled to Mexico recently, he said. Six of the U.S. cases were in San Diego and Imperial counties and the other two in Guadalupe County, Texas.
"It's really critically important we learn what is happening in Mexico," Besser said. "Sorting out which of the cases are caused by swine flu is an important public health question. . . . There is much uncertainty, more than anyone would like."
Cordova, the Mexican health secretary, said the virus was different because it wasn't striking the most vulnerable populations, but rather young adults and people who were otherwise healthy. That is potentially alarming because the 1918 influenza epidemic, which killed at least 20 million people worldwide, also struck the young and healthy.
"We have confirmation that this is a mutant of a virus that comes from pigs that . . . never had provoked an epidemic, that is, had never spread among humans," Cordova said.
Besser said the CDC had found that half of 14 samples from Mexico were positive for swine flu, with "similarity" to the strain that appeared in the U.S. "It's safe to say it's the same virus, from what we know," he said.
Investigators are looking into why the disease is so much more severe in Mexico, Besser said, and a CDC team will be traveling to Mexico.
In New York, health officials said Friday that about 75 students at a Queens high school had fallen ill with flu-like symptoms and were being tested to rule out the swine flu strain, the Associated Press reported.
U.S. health experts note that influenza deaths are common, most often among the very young and the elderly. In an average year in the United States, about 35,000 people die of the flu, and in bad years nearly twice that number.
The new swine virus is unlike any researchers have seen before. It appears to be a combination of segments from four viruses from three continents, with a human segment, an avian segment and pig segments.