Most flu victims are typically the youngest and the oldest, but health officials predict that a pandemic strain could spread quickest among school-age children.

"Rule No. 1 is to isolate people who are infected so they don't infect anyone else," says Julie Gerberding, director of the US Centers for Disease Control and Prevention.

But that will be just one step in a public-health response that also will involve global surveillance, vaccine development, complex systems for the distribution of antiviral drugs and travel restrictions.

Worldwide flu outbreaks occur when a virus emerges that people have never been exposed to, which has occurred three times in the past century, most recently in 1968. A strain of avian influenza, now spreading in Asia and Europe, has killed or prompted the destruction of millions of birds. It has infected 122 people and killed 62, raising concerns that it could begin spreading more widely and cause a pandemic.

Could Spread Quickest Among School-Age Children

The Department of Health and Human Service's new Pandemic Influenza plan warns that a flu pandemic could sicken one-third of the population and kill up to 1.9 million Americans. In ordinary years, the flu kills about 36,000 people in the USA alone. Health costs from a pandemic could total more than $181 billion, the plan says.

If the outbreak emerged overseas, the government may impose travel restrictions to affected countries, as many nations did during the 2003 SARS outbreak, according to the plan. In the USA, local health officials probably will resort to isolation of hospital patients and those ill at home and quarantine of people exposed to patients.

Most victims are typically the youngest and the oldest, but health officials predict that a pandemic strain could spread quickest among school-age children. To limit the spread of infection, health officials might quickly close schools and ban other public gatherings, according to the plan.

Because a vaccine will be unavailable in sufficient quantities for months, the plan recommends that antiviral drugs be prescribed first to patients and then to the healthcare workers and first responders who will come in contact with those who have the disease.

Months to Produce a Vaccine

Health and Human Services Secretary Michael Leavitt said in a briefing Wednesday that the government is working with drug companies to produce 20 million doses of a vaccine against the H5N1 influenza strain. No one knows whether the vaccine will work if H5N1 mutates sufficiently to begin spreading widely among humans.

"But it may provide some protection to first responders," he said.

It would take scientists and drug makers months to produce a vaccine that would protect people from the pandemic strain, he said.

The SARS outbreak that raced through Hong Kong, Singapore and Toronto in 2003 provides a compelling example of how difficult it is to contain a fast-spreading virus, says Susan Blumenthal, former assistant secretary of Health and Human Services, now at Georgetown and Tufts medical schools.

"SARS went to 30 countries on six continents," Blumenthal says. "We're fortunate it didn't go to Africa. There's hope that with worldwide cooperation we could stem the tide of an epidemic, but there's a lot of work to do."