In recent days, U.S. security officials have been urged, vehemently at times, to close the border with Mexico. Cruise lines have canceled stops along that country's coast. France asked the European Union to halt flights there. And one European health official even suggested that travel to the United States be avoided.
But here's the problem with such attempts to stop, or slow, the spread of the new H1N1 flu strain: Viruses don't grasp the concept of borders.
Attempting to seal off nations -- an inclination of many people and even some countries in the recent outbreak -- will prove futile not only now but in the future, experts say. Such measures cause unnecessary panic and economic harm, infectious disease experts say.
By the time a virus has been identified as a potential threat, it's probably already traveled far beyond its source, especially with today's mobile population.
Besides, obvious symptoms lag behind actual infection, meaning many people can be spreading the illness before they know they're ill.
"These actions really don't make sense," said Dr. Christian Sandrock, an infectious diseases expert at UC Davis Medical Center. "The benefits in lessening the spread of disease do not outweigh the economic and social problems these things cause."
Shutting down ports of entry between nations and attempting to control the movements of an area's residents are unlikely to halt transmission of a virus once it has spread beyond the first few cases of infection, epidemiologists say. Mathematical models and experience from the 2003 SARS outbreak support those findings.
"You really can't prevent the spread of these things by closing a border," said Dr. David Freedman, a spokesman for the International Society of Travel Medicine and a professor of medicine at the University of Alabama. "At best, physically closing a border could delay the spread of a disease by a few days. Travel restrictions are really ineffective."
Air travel has been shown to be the most influential mode of transmission in disease outbreaks, and an estimated 2,600 international flights land in the United States each day.
"Airplanes are silver tubes in which we put bugs from one country and send them to another country," said Dr. Sandro Galea, director of the Center for Global Health at the University of Michigan.
"There is little doubt that a pandemic is driven in large part by air travel. That is a simple fact that we have to deal with," he said. "But air travel is very much a part of our lives, and we want to keep it that way."
On Friday, an aircraft flying from Germany to Washington with 261 people on board was diverted to Boston after a female passenger complained of flu-like symptoms. But by the time the plane landed, the ill passenger, if infectious, most likely would have already transmitted the germs to others.
Instead of travel restrictions or border closures, the federal government is pursuing a more passive surveillance policy, Secretary of Homeland Security Janet Napolitano said Thursday.
Border agents and airport security personnel have been urged to watch for obviously ill travelers and to suggest they seek medical treatment and avoid travel if contagious.
Drastic actions such as quarantines aren't as helpful as they might appear. Viruses can spread even among people showing no symptoms, and the threat of detainment could make ill people less likely to seek medical help. That means they might stay in the broader population longer, spreading disease.
Federal emergency response policies call for using quarantine in a severe influenza epidemic, but a study published last year in the science journal PLoS One found that quarantining flu patients and their family members appears to have little impact. The study used a computer model to simulate the impact of the flu on a small U.S. city and the ramifications of various public health responses.
The study found that targeted containment measures -- use of antiviral medications, isolated school closings, treating sick people as needed -- would be more likely to slow the transmission of disease.
"We didn't find that you need quarantine as long as you apply those other measures," said Victoria Davey, one of the study's authors and deputy chief officer for public health and environmental hazards for the Department of Veterans Affairs in Washington. She is part of a team that advises the government on response to disease outbreaks.
Nor do real-world examples suggest that travel restrictions and border closures are especially effective.
During the SARS outbreak, Australia quarantined travelers returning from SARS-infected areas for several days, and countries such as Saudi Arabia and Malaysia restricted flights from Hong Kong, Canada and Taiwan. Canada screened travelers at all of its major airports.
But in a 2004 paper published in the journal Emerging Infectious Disease, officials at the World Health Organization concluded that those strategies did not delay transmission of the virus.