Cases of swine influenza have been confirmed in the United States, but so far the number is small and experts urge the public not to panic.

As of April 30, the Centers for Disease Control and Prevention (CDC), the federal government's infectious disease monitoring agency, reported 91 confirmed U.S. cases, and one death, a Mexican child visiting the United States. There are an estimated 1,900 cases in Mexico, the disease's epicenter, where at least 150 deaths are suspected to be related to the virus. On April 29, the World Health Organization (WHO) changed its global threat level to 5, its second-highest, and warned nations to prepare for a global flu pandemic. More than 100 schools in the U.S. have closed as a precaution.

Small children in Mexico City wear masks to protect them from the swine flu.

President Barack Obama advises people to be vigilant but calm. Swine flu "is obviously a cause for concern and requires a heightened state of alert, but it's not a cause for alarm," he said in remarks at the National Academy of Sciences on April 27. Obama added that U.S. health officials have "declared a public health emergency as a precautionary tool."

Still, World Health Organization (WHO) official Peter Cordingley warned, "There is a potential for this virus to become a pandemic." (An epidemic is a sudden outbreak of a disease in one area; a pandemic is a widespread epidemic.) In addition to the United States and Mexico, swine flu cases have been confirmed in Canada, Spain, New Zealand, Israel, and Great Britain. The WHO has upgraded its pandemic alert to phase 4 on a six-stage scale, which indicates that the virus can spread easily between humans and cause outbreaks. At the moment, officials are unsure how serious or widespread the swine flu will end up being.

A microbiologist in San Diego tests samples for the swine flu virus.

So far, no U.S. deaths have been linked to the outbreak of the swine influenza A (H1N1) virus. Most confirmed cases are in New York City, where some high school students at St. Francis Preparatory School in the Queens borough are believed to have caught the virus when they went to Mexico for spring break. (Those students are reportedly improving from mild cases.) Other cases have been confirmed in California, Kansas, Texas, and Ohio. St. Francis and schools in other areas where the virus has been confirmed have temporarily closed to help halt the flu's spread.

Swine flu usually isn't contagious to people. However, this strain of swine influenza has health officials worried. That's because it combines elements of the human flu (which is easily spread from one person to another) and bird flu (which is highly contagious, but usually just among birds) to create almost a superbug, according to Kristy Murray, assistant professor of epidemiology at the University of Texas School of Public Health in Houston. "The pig becomes a mixing vessel," she explains. The result is a brand-new virus "to which we are all basically vulnerable," Murray notes. "We have no immunity against it."

A "School Closed" sign hangs on the door of St. Francis Preparatory School in New York City, where eight students came down with the swine flu.

In 1918, another swine influenza virus caused a pandemic, killing approximately 50 million people throughout the world. "Swine flu is always one we worry about," says Murray. Still, for now, the current version doesn't seem as lethal as the 1918 virus.

Swine flu symptoms are similar to regular flu symptoms: fever, cough, body aches, headache, sore throat, chills, and exhaustion. Some people infected with the flu have also experienced vomiting and diarrhea. There is no vaccine against this version of influenza, but two prescription drugs - oseltamivir phosphate (Tamiflu) and zanamivir (Relenza) - seem to help treat it.

Unlike traditional flu bugs, which are most lethal in young children, people who are already sick, and the elderly, this strain seems to be particularly severe in people ages 25 to 45, according to Murray. Those people tend to have the healthiest and strongest immune systems, she points out. Somehow, the swine flu virus prompts an "over-response, an inflammatory cascade" that ends up damaging the body itself. Nevertheless, she says, "most cases in the United States have been typically mild."

Some basic steps can help cut down on the spread of any type of flu:

• Wash your hands with soap and water, for at least 20 seconds, after being out in public or using the bathroom, before touching food, and after sneezing or blowing your nose.
• Get enough healthy food, rest, and exercise to keep the immune system working well.
• Cover your mouth and nose with a tissue when you cough and sneeze. Toss the tissue away, and wash your hands afterward.
• You can't catch swine flu by eating hot dogs, bacon, ham, or other pork products. Instead, the virus spreads through droplets of mucus when people with the infection cough or sneeze.
• Call your doctor if you think you have flu symptoms. Don't go to a hospital emergency room unless the symptoms are severe.
• If you are infected with influenza, stay home from school and other activities.
• Although flu vaccine administered in 2008 won't help prevent the swine flu, getting the flu shot once a year is good protection in general.
  
  

For more on the swine flu outbreaks, check out these links:

• Kid- and teen-friendly information on swine flu.
• Swine Flu & You, from the Centers for Disease Control and Prevention.
• U.S. government's "one-stop shop" for flu information.
• Swine-flu related radio interviews with health experts.
• A blog post linking to up-to-date information on the disease's spread and number of cases and deaths worldwide.
• National Public Radio's swine-flu blog.