President Bush yesterday gave federal health officials standby power to quarantine anyone suspected of being infected with a new, sometimes fatal lung disease that is spreading around the world.

Bush issued an executive order adding severe acute respiratory syndrome, or SARS, to the list of illnesses for which the government can order someone held to prevent contagion.

It is the first time in 20 years that the list of diseases subject to federal quarantine has been expanded. In 1983, the disease caused by the deadly Ebola virus was added. The last time anyone was detained in a federal quarantine was in 1963, to prevent the spread of smallpox.

Federal health officials have said repeatedly that they have no plans to quarantine anyone and said yesterday that the move was just a precaution. But the order is the latest indication of the rising level of concern about SARS, which is suspected of having already sickened at least 115 Americans in 29 states, including two in Virginia.

Most of the cases have occurred among people who recently returned from parts of Asia where the disease is most common. But travelers have infected four family members and two health care workers upon their return. No one has died, and for unknown reasons the disease appears so far to be less severe in the United States.

Worldwide, SARS is believed to have struck at least 2,353 people in 18 nations. At least 84 have died.

If the disease were to spread widely in the United States, it "would have severe public health consequences," the executive order said.

Federal health officials have been meeting all planes, boat and cargo ships returning from Asia, isolating anyone with signs of SARS and warning other travelers to be on the lookout for symptoms.

During a briefing yesterday with reporters, James Hughes, director of the National Center for Infectious Diseases , said he would not be surprised to see more U.S. cases. "We are certainly wary and concerned about the possibility that this illness could spread to other countries, and we wouldn't be surprised if it did," Hughes said.

Bush saw the change to the quarantine list as a "logical precaution" that did not reflect any panic or new intelligence about the disease as an imminent threat in the United States, said White House press secretary Ari Fleischer.

"It's clearly a health concern, although in the United States it's not anywhere near as much as it is in some other parts of the world," Fleischer said.

Bush has been kept periodically informed about SARS and issued the order following a 40-minute briefing yesterday by Health and Human Services Secretary Tommy G. Thompson; Julie Gerberding, director of the Centers for Disease Control and Prevention; and Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.

A quarantine involves detaining someone who either has been infected or is suspected of being infected with a communicable disease to prevent it from spreading to others. Some patients with SARS have been isolated in this country to prevent them from infecting other people, but no one without symptoms has been quarantined.

Other countries, however, have taken more drastic measures. Hong Kong, which is among the hardest hit, has quarantined hundreds of people to try to bring the epidemic under control, as has Singapore.

Prior to the order, the list of diseases that can trigger a federal quarantine included cholera, diphtheria, infectious tuberculosis, plague, smallpox, yellow fever, and viral hemorrhagic fevers such as Ebola, Lassa and Marburg.

Thompson said he asked Bush to amend the order in the unlikely event an infected person arrives in the country and refuses treatment.

"Ninety-five percent of people will do it voluntarily because it is in their own best interest," he said in an interview. "This executive order would give us the authority for those individuals who do not want to stop and get the medical attention they need."

Thompson would have liked the authority several days ago when a woman arriving on a plane from China showed SARS symptoms but would not see a doctor or go into isolation as officials requested. "She refused, got on a train, got very sick and had to get off," he said. "We didn't have any way to stop her." The woman recovered.

Earlier this week in Atlanta, Thompson said there may come a time when the United States will need to consider quarantines, but "we are not there yet." States already have broad authority to quarantine, he added.

If health officials felt they needed to quarantine an entire building or community -- as has happened overseas -- Thompson said he would contact the governor and request the state take that action. "Hopefully we would not have to do anything like that, but anything is possible," he said.

SARS causes a flu-like syndrome, marked by symptoms such as high fever, cough and breathing problems. At least 80 percent of patients recover, but the rest get sicker, with many developing pneumonia and some requiring a ventilator to breathe. About 3.5 percent die. There is no vaccine or known effective treatment.

Most of the cases have been in southern China, where the disease is believed to have first emerged in November before spreading to Hong Kong. From there, air travelers spread the disease around the world, prompting a global health emergency to contain the illness.

A new ABC/Washington Post poll found that 74 percent of Americans are following the SARS story closely, 38 percent are worried about the disease, and 35 percent think SARS poses a greater danger to themselves or their family than a terrorist attack.

Staff writers Mike Allen, Ceci Connolly and Richard Morin contributed to this report.