Mexico's president told citizens on Wednesday to stay home for a five-day partial shutdown of the economy, after the World Health Organization raised its alert level and said a swine flu pandemic was imminent.

In his first televised address since the crisis erupted last week, President Felipe Calderon told Mexicans to stay home with their families. The country will suspend non-essential work and services, including some government ministries, from May 1-5.

"There is no safer place than your own home to avoid being infected with the flu virus," Calderon said.

Mexico is taking the drastic step after another 17 deaths were potentially linked to swine flu, bringing the total to as many as 176.

Essential services such as transport, supermarkets, trash collection and hospitals will remain open.

The swine flu outbreak continued to fan out across the United States on Wednesday, with confirmed cases in at least 11 states and possible cases in several more. American deaths have not become widespread despite confirmation that a Mexican toddler who was visiting Texas with his family died from the flu.

Illinois officials cited nine "likely cases," most of them in the Chicago area, and three schools have been shut down. Washington state health officials report six probable cases, and Maryland health officials are awaiting lab tests to confirm six cases considered "likely."

In California, dozens of Marines were confined after one came down with the disease. Some 100 schools were closed, and more might need to be shut down temporarily.

President Obama pledged "great vigilance" in dealing with the situation as the total confirmed cases in the U.S. rose to nearly 100, with many more suspected.

The Geneva-based World Health Organization sounded its own ominous alarm, raising its alert level to one notch below a full-fledged global pandemic. Said WHO Director General Margaret Chan: "It really is all of humanity that is under threat during a pandemic."

Dr. Richard Besser, acting chief of the Centers for Disease Control and Prevention, said there were confirmed cases in 10 states, including 51 in New York, 16 in Texas and 14 in California. The CDC also counted scattered cases in Kansas, Massachusetts , Michigan, Arizona, Indiana, Nevada and Ohio.

At least 10 other states from Rhode Island to Nevada and the Carolinas to Illinois are examining either possible or "probable" cases of swine flu in their states.

State officials in Maine said laboratory tests had confirmed three cases in that state, not yet included in the CDC count.

And the Pentagon said a Marine at the Twentynine Palms base in California had been confirmed to be ill with swine flu and was isolated, along with his roommate. A Marine spokesman at the Pentagon, Major David Nevers, said the sick Marine was doing well and his condition continued to improve. Nevers said about 30 others who had been in contact with the sick Marine would be held apart for five days as well as to see if they show symptoms.

Dr. Manny Answers Your Questions on Swine Flu.

In Mexico, where the flu is believed to have originated, officials said Wednesday that the disease was now suspected in nearly 2,500 illnesses. The Mexican health ministry spokesman said the country has had 17 more possible swine flu deaths since Tuesday, bringing the new total up to 176.

The first death in the United States from the flu was a Mexico City toddler who traveled to Texas with his family to visit relatives.

Texas' health director, Dr. David Lakey, told a news conference that it was "highly likely" that the boy contracted the disease in Mexico before his trip to the U.S.

Officials in Brownsville were trying to trace his family's trip to find out how long they were in the area, who they visited and how many people were in the group, said Cameron County Judge Carlos Cascos.

The boy, who was 23 months old, had "underlying health issues" before he flew to Matamoros, Mexico, on April 4 and crossed into Brownsville to visit relatives, state health officials said.

He developed flu symptoms four days later and was taken to a Brownsville hospital April 13 and transferred the following day to Texas Children's Hospital in Houston, where he died Monday night.

The U.S. Centers for Disease Control and Prevention on Wednesday confirmed that he had been infected with the swine flu virus. The cause of the death was pneumonia caused by the virus, Cascos said.

Texas called off all public high school athletic and academic competitions at least until May 11 due to the outbreak.

Despite calls from many U.S. lawmakers for tightening controls over the Mexico-US border, Obama and his deputies ruled out that option.

At a prime-time news conference, Obama said health officials weren't recommending closing the border with Mexico.

That, he said, "would be akin to closing the barn door after the horses are out, because we already have cases here in the United States."

In an interview with Dr. Manny Alvarez on Wednesday, managing editor of health at FOXNews.com, Dr. Dalilah Restrpo, an infectious disease specialist at St. Luke's-Roosevelt in New York City drove home the same point.

"It's always an tempting strategy to close the door, back off and not let anybody else in, but it doesn't work," she told Alvarez.

"It doesn't work because this type of disease is something where the period of infectiousness is behind you," she continued. "By the time the symptoms arrive — it's already too late. At this point, closing the border would make no sense. There are cases all over the world and the virus is already here."

Instead of closing the border, Obama said, his administration had ramped up screening efforts and made sure needed medical supplies were on hand.

"The key now is to just make sure we are maintaining great vigilance, that everybody responds appropriately when cases do come up," he said. "And individual families start taking very sensible precautions that can make a huge difference."

In fact, customs agents have delayed 49 people at the border because of flulike symptoms, and 41 have been cleared so far. Test results on the other eight were not complete.

Laboratory testing showed the new virus was treatable by the anti-flu drugs Tamiflu and Relenza, and the first shipments from a federal stockpile arrived Wednesday in New York City and several other locations. The government was shipping to states enough medication to treat 11 million people as a precaution. All states should get their share by May 3.

No shortages had been reported — there was plenty in regular pharmacies, federal health officials said.

VIDEO: How you can prevent the spread of swine flu.

The disease is not spread by eating pork and U.S. officials appeared to go out of their way on Wednesday to not call the strain "swine flu." Obama called the bug the "H1N1 virus," and other administration officials followed his lead.

"The disease is not a food-borne illness," Rear Adm. Anne Schuchat, CDC's interim science and public health deputy direct, told the Senate Homeland Security Committee.

She said the strain is particularly worrisome because "it's a virus that hasn't been around before. The general population doesn't have immunity from it."

Emergency Response

There is no vaccine to prevent infection but U.S. health officials aim to have a key ingredient for one ready in early May, the big step that vaccine manufacturers are awaiting. But even if the World Health Organization ordered up emergency vaccine supplies — and that decision hasn't been made yet — it would take at least two more months to produce the initial shots needed for human safety testing.

"We're working together at 100 miles an hour to get material that will be useful," Dr. Jesse Goodman, who oversees the Food and Drug Administration's swine flu work, told The Associated Press.

Obama asked Congress for $1.5 billion in emergency funds to help build more drug stockpiles and monitor future cases, as well as help international efforts to avoid a full-fledged pandemic.

In Washington, Homeland Security Secretary Janet Napolitano was questioned closely by senators about whether the U.S. should close its border with Mexico, repeated the administration's position that questioning of people at borders and ports of entry was sufficient for now and said closing borders "has not been merited by the facts."

In a bit of good news, Mexico's health secretary, Jose Cordova, late Tuesday called the death toll there "more or less stable."

Swine Flu vs. Seasonal Influenza

“The illness is consistent with seasonal influenza and has generally the same symptoms of seasonal influenza,” Fukuda said. “Our information to date shows that the infection can range from very mild, requiring no hospitalization and recovering after a few days, to fatal, which is also in keeping with seasonal influenza."

Fukuda said there is no evidence that the virus is spreading from pigs or that it is unsafe to eat pork.

Authorities sought to keep the crisis in context: Flu deaths are common around the world. In the U.S. alone, the CDC says about 36,000 people a year die of flu-related causes.

Children, especially those younger than age 5, are particularly vulnerable to flu and its complications, and every year children die from seasonal flu.

According to the CDC, more than 20,000 children younger than age 5 are hospitalized every year because of seasonal flu. In the 2007-08 flu season, the CDC received reports that 86 children nationwide died from flu complications.

As of April 11, CDC had received reports of 53 seasonal flu-related deaths in children during the current seasonal flu season.

Still, the CDC calls the new strain a combination of pig, bird and human viruses for which people may have limited natural immunity.

Hence the need for a vaccine. Using samples of the flu taken from people who fell ill in Mexico and the U.S., scientists are engineering a strain that could trigger the immune system without causing illness. The hope is to get that ingredient — called a "reference strain" in vaccine jargon — to manufacturers around the second week of May, so they can begin their own laborious production work, said CDC's Dr. Ruben Donis, who is leading that effort.

The first doses of a swine-flu vaccine could be available about 15 weeks after the World Health Organization decides what kind of vaccine it wants companies to produce, Sanofi-Aventis SA's chief executive said in an interview with the Wall Street Journal.

The Associated Press and Reuters contributed to this report.

Related:

Swine Flu Timeline

Swine Flu Q & A

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