The spread of swine flu appears to be slowing in Mexico even as it continues to blaze a path across the U.S.

Although Mexico's top medical officer voiced optimism Thursday as the virus stabilizes in the hardest hit country, the World Health Organization cautioned that there is no evidence the global outbreak is over.

Mexican Health secretary Jose Angel Cordova told a news conference he hoped the trend will continue and that a vaccine would be available in six months. European health ministers said they would speed efforts to develop such a vaccine.

In the U.S., 130 cases have been confirmed in 20 states, although state health experts say the count is likely much higher. Among the U.S. cases confirmed by the Centers for Disease Control and Prevention are 51 in New York, 16 in Texas and 14 in California, as well as scattered cases in Kansas, Massachusetts, Michigan, Arizona, Indiana, Nevada, Ohio, Maine and South Carolina.

State officials also confirmed cases in Minnesota, Georgia, New Jersey, Delaware, Utah, New Jersey, Virginia and Colorado. On Thursday night, Illinois health officials announced the number of probable swine flu cases in the state had more than doubled to 41.

About 300 of the nation's 132,000 schools have closed, including schools on both coasts and in the nation's heartland. More are likely to shut their doors in coming days. Additionally, high school, college and professional sporting events have been called off nationwide due to increasing fears.

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Nebraska's chief medical officer says federal tests have confirmed the first case of swine flu in the state. It involves a California man in his 40s who's vacationing in the Omaha area. Health officials have said the infection ultimately will be reported as a California case because the CDC counts cases of infectious disease by residence.

Wisconsin Gov. Jim Doyle declared a public health emergency after two more probable cases of swine flu were identified in Wisconsin Thursday.

In a news release Thursday, Dr. Joann Schaefer also said two more probable cases in Nebraska have been detected.

Thirty-nine Marines were confined to their base in California after one came down with the disease. The WHO has raised the total number of confirmed cases worldwide to 236.

And a pediatrician in Washington state saw 22 patients with flu-like symptoms before she developed serious symptoms and went to the emergency room.

The Pennsylvania Department of Health says it continues to find people who it considers to be probable swine flu cases. It is investigating three new possible cases.

Illinois health officials say the number of probable swine flu cases in the state has more than doubled to 41.

Virginia officials say they have turned up two confirmed cases of swine flu in the state. In both cases, the patients - who have since recovered - had traveled to Mexico.

A security aide helping with arrangements during President Barack Obama's recent trip to Mexico became sick with flu-like symptoms and three members of his family later contracted probable swine flu, the White House said Thursday.

The disclosure from press secretary Robert Gibbs comes days after the White House played down risks to the U.S. delegation on the two-day trip that started April 16. Gibbs remained steadfast that the president was never at risk of contracting the flu, which has quickly spread across the globe.

It was also learned that a Mexican toddler who visited Texas with his family and died Monday night in Houston, spent a day with his family shopping at a huge Houston indoor mall the day before he began to show symptoms, said Cameron County Judge Carlos Cascos, who interviewed the boy's family.

Vice President Joe Biden is arguing that there would be no practical benefit in shutting down the country's border with Mexico.

The acting head of the Centers for Disease Control and Prevention, Dr. Richard Besser, agreed with the administration's sentiment Thursday, saying there could be "negative consequences" to this option for containing the spreading swine flu virus. In a news conference Thursday, Bresser said new cases of swine flu continue to occur in the U.S. with the latest case on April 26.

Slippery Rock University will hold a separate graduation for 22 students who recently returned from Mexico.

So far, the students are in good health, but school officials fear they could have been exposed to the swine flu. Commencement falls in the middle of an incubation period during which the students have been advised to limit contact with others.

The students were in Mexico completing their student-teacher requirements. The flu's spread prompted schools to close there and the students returned Tuesday, two days early. Saturday's ceremony for the students will be private. In all, the school is awarding more than 1,300 diplomas.

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Meanwhile, the World Health Organization said it plans to no longer call the deadly flu "swine flu" to avoid confusion over the risk from pigs and eating pork. Health officials have stressed repeatedly that the disease is being transmitted human to human and that there is no risk from eating pork.

WHO spokesman Dick Thompson says the name change comes after the agriculture industry and the U.N. food agency expressed concerns that the term "swine flu" was misleading consumers and needlessly causing countries to order the slaughter of pigs.

He told reporters in Geneva "we're going to stick with the technical scientific name H1N1 influenza A."

Several countries have put a ban a pork imports. Egypt went as far as to slaughter several thousand pigs, and hog futures fell for the sixth time in seven sessions on Thursday because of continued speculation that people will stop eating pork.

On Wednesday, the WHO boosted its alert level to one level below a full-fledged pandemic. The new Phase 5 alert, indicating a pandemic could be imminent as the virus spread further in Europe, prompted Mexico to announce the partial May 1-5 shutdown, Mexican Health Secretary Jose Cordova said late Wednesday.

The World Health Organization's flu chief, reacting to similar comments from other Mexican officials, cautioned that case numbers often go up and down, and said the WHO had yet to see concrete evidence that swine flu, believed to have killed 168 people in Mexico, was leveling off.

"It's a mixed pattern out there," Fukuda said. "What's happening in one part of the country is not necessarily what's happening in another part of the country."

The virus, a mix of pig, bird and human genes to which people have limited natural immunity, has also spread to Canada, New Zealand, Britain, Germany, Spain, Israel and Austria.

"It really is all of humanity that is under threat during a pandemic," WHO Director General Margaret Chan said in Geneva. "We do not have all the answers right now, but we will get them."

The U.S., the European Union and other countries have discouraged nonessential travel to Mexico. Some countries have urged their citizens to avoid the United States and Canada as well. Health officials said such bans would do little to stop the virus.

Where Did Virus Originate?

Medical detectives have not pinpointed where the outbreak began. Scientists believe that somewhere in the world, months or even a year ago, a pig virus jumped to a human and mutated, and has been spreading between humans ever since.

China has gone on a rhetorical offensive to squash any suggestion it's the source of the swine flu after some Mexican officials were quoted in media reports in the past week saying the virus came from Asia and the governor of Mexico's Veracruz state was quoted as saying the virus specifically came from China.

One of the deaths in Mexico directly attributed to swine flu was that of a Bangladeshi immigrant, said Mexico's chief epidemiologist Miguel Angel Lezana.

Lezana said the unnamed Bangladeshi had lived in Mexico for six months and was recently visited by a brother who arrived from Bangladesh or Pakistan and was reportedly ill. The brother has left Mexico and his whereabouts are unknown, Lezana said. He suggested the brother could have brought the virus from Pakistan or Bangladesh.

By March 9, the first symptoms were showing up in the Mexican state of Veracruz, where pig farming is a key industry in mountain hamlets and where small clinics provide the only health care.

The earliest confirmed case was there: a 5-year-old boy who was one of hundreds of people in the town of La Gloria whose flu symptoms left them struggling to breathe.

Neighbors of the inspector, Maria Adela Gutierrez, said Wednesday that she fell ill after pairing up with a temporary worker from Veracruz who seemed to have a very bad cold. Other people from La Gloria kept going to jobs in Mexico City despite their illnesses, and could have infected people in the capital.

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.

Obama said his administration has made sure that needed medical supplies are on hand and he praised the Bush administration for stockpiling 50 million doses of antiviral medications

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.

The Associated Press and Reuters contributed to this report.

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