WASHINGTON — President Bush's plan for dealing with a flu pandemic warns that the federal government won't be able to bail out communities reeling from illness and economic upheaval, and it calls on businesses and individuals to take steps now to keep vital services running.

The updated plan, released Wednesday, stresses basic human needs such as medical care and food but doesn't address some major hurdles — how to meet those needs if massive absenteeism stops transportation by closing oil refineries or crashes the Internet so workers can't telecommute.

"Our efforts require the participation of, and coordination by, all levels of government and segments of society," Bush said in a letter to Americans unveiling his updated national pandemic response strategy.

"No less important will be the actions of individual citizens, whose participation is necessary to the success of these efforts."

Influenza pandemics strike every few decades when a never-before-seen strain arises. It's impossible to predict when the next will occur, or its toll. But last fall, amid concern that the Asian bird flu might lead to one if it starts spreading easily from person to person, Bush proposed a $7.1 billion, multi-year strategy to prepare for the next pandemic.

At the plan's core is stockpiling enough bird-flu vaccine for 20 million people, plus anti-flu medications and other key medical supplies, to provide some protection while manufacturers race to brew a pandemic-specific inoculation.

Wednesday's report updates Bush's initial plan, outlining exactly which government agency is responsible for some 300 additional tasks. It also provides details, beyond health care, of changes Americans could expect in how they travel, work and conduct day-to-day activities during a severe pandemic.

The report's big message: "Local communities will have to address the medical and nonmedical impacts of the pandemic with available resources." That's because the federal government won't be able to offer the kind of aid expected after hurricanes or other one-time, one-location natural disasters, it says.

U.S. borders won't be sealed after outbreaks abroad, the report says. That would fail to keep out a pandemic — people can spread flu a full day before they show symptoms. Instead, the goal will be to slow influenza's march, starting by screening international travelers for signs of infection and quarantining possibly ill passengers.

As U.S. infections mount, people will be asked to stay away from crowds, and cancel nonessential travel. Patients' families would be temporarily isolated. Schools in affected communities would close.