By Donald G. McNeil Jr.

Other nations imposed travel bans or made plans to quarantine air travelers as confirmed cases also appeared in Mexico and Canada and suspect cases emerged elsewhere.

Top global flu experts struggled to predict how dangerous the new A (H1N1) swine flu strain would be as it became clear that they had too little information about Mexico’s outbreak — in particular how many cases had occurred in what is thought to be a month before the outbreak was detected, and whether the virus was mutating to be more lethal, or less.

“We’re in a period in which the picture is evolving,” said Dr. Keiji Fukuda, deputy director general of the World Health Organization. “We need to know the extent to which it causes mild and serious infections.”

Without that knowledge — which is unlikely to emerge soon because only two laboratories, in Atlanta and Winnipeg, Canada, can confirm a case — his agency’s panel of experts was unwilling to raise the global pandemic alert level, even though it officially saw the outbreak as a public health emergency and opened its emergency response center.

As a news conference in Washington, Homeland Security Secretary Janet Napolitano called the emergency declaration “standard operating procedure,” and said she would rather call it a “declaration of emergency preparedness.”

“It’s like declaring one for a hurricane,” she said. “It means we can release funds and take other measures. The hurricane may not actually hit.”

American investigators said they expected more cases here, but noted that virtually all so far had been mild and urged Americans not to panic.

The speed and the scope of the world’s response showed the value of preparations made because of the avian flu and SARS scares, public health experts said.

The emergency declaration in the United States lets the government free more money for antiviral drugs and give some previously unapproved tests and drugs to children. One-quarter of the national stockpile of 50 million courses of antiflu drugs will be released.

Border patrols and airport security officers are to begin asking travelers if they have had the flu or a fever; those who appear ill will be stopped, taken aside and given masks while they arrange for medical care.

“This is moving fast and we expect to see more cases,” Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said at the news conference with Ms. Napolitano. “But we view this as a marathon.”

He advised Americans to wash their hands frequently, to cover coughs and sneezes and to stay home if they felt ill; but he stopped short of advice now given in Mexico to wear masks and not kiss or touch anyone. He praised decisions to close individual schools in New York and Texas but did not call for more widespread closings.

Besides the eight New York cases, officials said they had confirmed seven in California, two in Kansas, two in Texas and one in Ohio. The virus looked identical to the one in Mexico believed to have killed 103 people — including 22 people whose deaths were confirmed to be from swine flu — and sickened about 1,600. As of Sunday night, there were no swine flu deaths in the United States, and one hospitalization.

Other governments tried to contain the infection amid reports of potential new cases including in New Zealand and Spain.

Dr. Fukuda of the W.H.O. said his agency would decide Tuesday whether to raise the pandemic alert level to 4. Such a move would prompt more travel bans, and the agency has been reluctant historically to take actions that hurt member nations.

Canada confirmed six cases, at opposite ends of the country: four in Nova Scotia and two in British Columbia. Canadian health officials said the victims had only mild symptoms and had either recently traveled to Mexico or been in contact with someone who had.

Other governments issued advisories urging citizens not to visit Mexico. China, Japan, Hong Kong and others set up quarantines for anyone possibly infected. Russia and other countries banned pork imports from Mexico, though people cannot get the flu from eating pork.

In the United States, the C.D.C. confirmed that eight students at St. Francis Preparatory School in Fresh Meadows, Queens, had been infected with the new swine flu. At a news conference on Sunday, Mayor Michael R. Bloomberg said that all those cases had been mild and that city hospitals had not seen a surge in severe lung infections.

On the streets of New York, people seemed relatively unconcerned, in sharp contrast to Mexico City, where soldiers handed out masks.

Hong Kong, shaped by lasting scars as an epicenter of the SARS outbreak, announced very tough measures. Officials there urged travelers to avoid Mexico and ordered the immediate detention of anyone arriving with a fever higher than 100.4 degrees Fahrenheit after traveling through any city with a confirmed case, which would include New York.

Everyone stopped will be sent to a hospital for a flu test and held until it is negative. Since Hong Kong has Asia’s busiest airport hub, the policy could severely disrupt international travel.

The central question is how many mild cases Mexico has had, Dr. Martin S. Cetron, director of global migration and quarantine for the Centers for Disease Control, said in an interview.

“We may just be looking at the tip of the iceberg, which would give you a skewed initial estimate of the case fatality rate,” he said, meaning that there might have been tens of thousands of mild infections around the 1,300 cases of serious disease and 80 or more deaths. If that is true, as the flu spreads, it would not be surprising if most cases were mild.

Even in 1918, according to the C.D.C., the virus infected at least 500 million of the world’s 1.5 billion people to kill 50 million. Many would have been saved if antiflu drugs, antibiotics and mechanical ventilators had existed.

Another hypothesis, Dr. Cetron said, is that some other factor in Mexico increased lethality, like co-infection with another microbe or an unwittingly dangerous treatment.

Flu experts would also like to know whether current flu shots give any protection because it will be months before a new vaccine can be made.

There is an H1N1 human strain in this year’s shot, and all H1N1 flus are descendants of the 1918 pandemic strain. But flus pick up many mutations, and there will be no proof of protection until the C.D.C. can test stored blood serum containing flu shot antibodies against the new virus. Those tests are under way, said an expert who sent the C.D.C. his blood samples.

Reporting was contributed by Sheryl Gay Stolberg from Washington, Jack Healy from New York, Keith Bradsher from Hong Kong and Ian Austen from Ottawa.