In the event of a flu pandemic, people exposed to the virus could be quarantined in their homes, kept off planes or intercepted at airports and other ports of entry, as outlined in the federal government's preparedness plan.
Such measures are contained in a 396-page plan issued yesterday by the Department of Health and Human Services, a day after President Bush gave the broad outline.
The document says health authorities don't know when or whether a pandemic might strike, or how severe it might be. It suggests two possibilities, a moderate pandemic like the ones in 1957 and 1968 or a severe pandemic like the one that swept the world in 1918-1919.
In either case, flu could sicken 90 million Americans and cause 45 million to seek treatment.
That is where the similarities end. A moderate pandemic could result in the hospitalization of 865,000 people and the deaths of 209,000; a severe one could send 9.9 million people to hospitals and kill 2 million.
Despite the uncertainties, said Health and Human Services Secretary Michael O. Leavitt, the nation must prepare for the worst. "Think of the world as a vast forest susceptible to fire," he said. "With a pandemic, it only takes a spark to create a pandemic."
The concern is that a strain of bird flu that has arisen in Southeast Asia could touch off a worldwide pandemic. That could happen if the virus acquires the ability to spread from person to person.
All but a few of the 120 reported human cases have grown out of contact with poultry. Sixty people have died.
The government's plan calls on Congress to appropriate $7.1 billion to stockpile vaccines and anti-viral drugs, spur faster vaccine production and improve detection systems in the United States and overseas.
To curb the disease, state and local governments might have to isolate or quarantine those exposed to the virus.
Isolation refers to the separation of sick people until they are no longer infectious, which means up to two weeks, experts say. Quarantines, which separate people who have been exposed to an illness but are not sick, could last a few days or as long as 10 days to prevent spread of pandemic flu, according to the plan.
"A person's residence is generally the preferred setting for quarantine" because that would be least disruptive, the document says.
Local authorities might have to establish facilities for people whose homes are not suitable or who are traveling when exposed. Such places must have adequate bathrooms, telephones, food and medical staff, the plan says.
Isolation and quarantine are frequently used, but never before on as large a scale as the new plan projects.
Dr. Michelle Gourdine, Maryland's deputy secretary for public health services, said people are frequently isolated in hospitals while suffering from tuberculosis and other highly contagious diseases.
During the SARS epidemic in 2003, people who had been in contact with patients overseas were isolated until it became clear that they were not ill.
"As far as quarantines, we always try to opt for the least restrictive option," Gourdine said.
Quarantines might not be useful if large numbers of people have been exposed before the epidemic has been detected, she said.
Dr. Donald Burke, head of the immunization research program at the Johns Hopkins Bloomberg School of Public Health, said quarantines could fall short in controlling a virus that people transmit as soon as a day after being exposed. Additionally, some people who are highly infectious might not know they are endangering others.
"If you take the 1918 epidemic as any lesson, many of the cases might not be highly symptomatic and [people] may be ambulatory and only mildly ill," he said.
Dr. Thomas C. Quinn, an infectious-disease specialist at the Johns Hopkins School of Medicine, said quarantines can be helpful in limiting an epidemic.
"You can take a massive epidemic and reduce it to a minor one," Quinn said. "It does limit the number of people who come into contact with an infected person. It's a numbers game."
Influenza is one of the fastest-moving viral infections, he said.
The federal plan notes that quarantines can be effective when they are voluntary, with more than 90 percent of people complying if the need is made clear.
"Public health authorities not only expect but generally get compliance with quarantine and isolation measures from a large measure of the population," said James Hodge, who heads the Center for Law and the Public's Health at the Bloomberg school.
The plan says that travel restrictions may be imposed on people suspected of carrying the virus. Those already en route might be intercepted at airports or docks. These passengers would be examined and possibly transported to hospitals or quarantine facilities.
The U.S. Centers for Disease Control and Prevention has 18 quarantine stations at international airports, ports and border crossings to handle passengers flagged by flight crews and customs inspectors.
Once a pandemic is in full force, authorities might slow its spread in the United States by limiting or canceling nonessential travel to unaffected regions. Mass transit systems might be closed.