WEDNESDAY, April 29 (HealthDay News) -- A 23-month-old Mexican boy who had traveled to Houston for medical treatment has become the first fatality in the United States in the spreading swine flu outbreak, federal health officials said Wednesday morning.

Dr. Richard Besser, acting director of the U.S. Centers for Disease Control and Prevention, confirmed the death.

"A child has died from the H1N1 virus," he said. "As a parent and a pediatrician, my heart goes out to the family."

The boy was not a U.S. citizen, said Kathy Barton, a spokeswoman for the Houston Department of Health and Human Services, adding she could provide no other details, CNN reported.

Also Wednesday, the World Health Organization raised the influenza epidemic level from 4 to 5, signifyng that a pandemic is imminent, and urged countries to implement their pandemic plans, CNN said.

And late in the day, the Pentagon confirmed that a Marine at the Twentynine Palms base in Southern California has come down with swine flu and is under quarantine, along with a roommate and about 30 others with whom he had contact. Marine spokesman Maj. David Nevers told the Associated Press that the man's condition is improving.

Meanwhile, President Barack Obama said Wednesday that U.S. public health officials were recommending that schools with confirmed or suspected cases of swine flu "should strongly consider temporarily closing so that we can be as safe as possible," the AP reported.

At a press briefing Wednesday morning, Besser said there were now 91 confirmed cases of infection with the never-before-seen virus in 10 states, with the one death. Sixty-four percent of the cases involve people under age 18, but patients range in age from 8 to 81, he said.

Border surveillance has been increased to actively look for cases of infection, he said.

Kathleen Sebelius, the new secretary of U.S. Health and Human Services (HHS), said at the briefing that, "while we still don't know what this virus will do, we expect to see more cases, more hospitalizations and, unfortunately, we are likely to see additional deaths from the outbreak."

"Currently, the FDA [U.S. Food and Drug Administration] and the CDC are developing virus reference strains -- the information that is necessary to develop a vaccine," Sebelius said. "Today, there are a series of steps that HHS is taking in vaccine development. The process is more speedy than it has ever been before."

The earliest a vaccine could be ready is this fall, said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.

During a press briefing Tuesday, Besser had said that the cases of infection found in the United States so far continued to be mild, but more severe cases were expected, and "as we move forward, I fully expect we will see deaths."

Many of the swine flu cases in the United States come from a New York City high school that had previously reported 18 cases of the infectious disease, Besser said.

Besser said the incubation period for the U.S. cases is two to seven days, which, he said, "is typical for what you see with an influenza virus."

The majority of new cases in New York continued to come at St. Francis Preparatory School in Queens. Some students who have come down with the infection had been to Mexico -- believed to be the source of the outbreak -- during a spring break trip to Cancun, the AP reported.

Texas has postponed all public high school sports and academic competitions at least until May 11 due to the outbreak.

As with the previously tested strains of the swine flu virus, new testing found that the pathogen remains susceptible to the two common antiviral drugs Tamiflu and Relenza, according to an April 28 dispatch from the CDC's Morbidity and Mortality Weekly Report.

The new flu strain is a combination of pig, bird and human viruses, prompting worries from health officials that humans may have no natural immunity to the virus.

Meanwhile, the AP reported that the epidemic had crossed new borders, with the first cases confirmed in the Middle East and the Asia-Pacific region. Canada, Austria, New Zealand, Israel, Spain, Britain and Germany also have reported cases of swine flu sickness. Deaths reported so far have been limited to Mexico, and now the United States.

On Tuesday, Cuba and then Argentina suspended flights to and from Mexico, becoming the first countries to impose a travel ban, even though the World Health Organization (WHO) has said such bans were ineffective because the virus has already taken root in more than one country, the AP said.

In Mexico, meanwhile, the toll from the swine flu epidemic appeared to be stabilizing, health officials there said late Tuesday, with only seven more suspected deaths, CBS News reported.

The virus is suspected in 159 deaths and 2,498 illnesses across Mexico, said Health Secretary Jose Cordova, who called the death toll "more or less stable." He said only 1,311 suspected swine flu patients remain hospitalized, a sign that treatment works for people who get medical care quickly, the news network reported.

On Monday, U.S. officials said they were tightening their travel advisory to Mexico, recommending that all non-essential travel to that country be avoided.

The United States has stepped up checks of people entering the country by air, land and sea, looking for signs of infection, and the CDC plans to distribute "yellow cards at ports of entry," Besser said Monday.

"These will provide information on swine flu, so that people coming into the United States will have information about this outbreak -- what to do if they become sick, what things they can do to prevent the likelihood that they will become sick," he said.

He also said U.S. officials were questioning border visitors about their health, looking for signs of possible infection.

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