Health officials don't know enough about swine flu right now to say what the precise incubation period is, but if it's similar to other flu, people are likely able to spread it before they're sick.
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Pigs wait for their food in a farm in Makassar April 29, 2009. The United Nations' food agency said on April 28 its was mobilising its animal health experts to check if the new strain of flu virus widely described as swine flu is really directly linked to pigs. [Agencies]
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"WHO does not recommend closing of borders and does not recommend restrictions of travel," said Dr. Keiji Fukuda, the Geneva-based organization's flu chief. "From an international perspective, closing borders or restricting travel would have very little effect, if any effect at all, at stopping the movement of this virus."
Nor will killing pigs, as Egypt began doing Wednesday, infuriating pig farmers who blocked streets and stoned Health Ministry workers' vehicles in protest. While pigs are banned entirely in some Muslim countries because of religious dietary restrictions, they are raised in Egypt for consumption by the country's Christian minority.
Unlike bird flu, where the H5N1 strain that spread to humans was widespread in bird populations and officials worried about people's exposure to infected birds, WHO says there is no similar concern about pigs -- and no evidence that people have contracted swine flu by eating pork or handling pigs.
"There is no association that we've found between pigs and the disease in humans," WHO spokesman Dick Thompson said.
But that hasn't stopped governments from banning pig products. Macedonia ordered a halt to all live pig imports and on Tuesday, Mexico City closed down all its popular streetside taco stands for at least a week.
Dr. Nikki Shindo, a WHO flu expert, said the agency will consider requests to stop calling the disease swine flu, since the virus is not food-borne and has nothing to do with eating pork.
China, Russia and Ukraine are among countries that have banned pork imports from Mexico and parts of the United States affected by swine flu.
But some anti-flu measures have merit, such as Obama's admonition Wednesday that more American schools might have to be closed temporarily if swine flu cases spread. Already tens of thousands of students in Texas, New York, California, Chicago and elsewhere are out of school.
The WHO said closing schools and public places, along with banning or restricting mass gatherings, can be a way to contain the spread of disease. Epidemiologists call it "social distancing," and the idea is simple: If you keep people who have the virus away from others, you can stop the chain of transmission.
"That's a technique we would be recommending in a pandemic," said WHO's Thompson. "We would recommend it to nations as a useful technique to be applied given the special circumstances of each nation."