The virus that carries foot-and-mouth disease is so infectious that it could slip past defenses meant to keep it out of the United States, experts say.

The foot-and-mouth virus could enter the country on plant matter, in the luggage of travelers returning from abroad or even on the soles of their shoes. It's not dangerous to humans but is highly dangerous to many farm animals. Once established in the United States it could probably be eradicated, but only after causing great economic damage.

"Foot-and-mouth is probably the most communicable disease in the world, period," said Max Coats, deputy director for animal health programs at the Texas Animal Health Commission.

Most viruses can spread only through fairly close contact, such as coughs or sneezes. But foot-and-mouth can hitch a ride on shoes, clothing or vehicles. Wind can carry it dozens or hundreds of miles.

Last year in Japan, where foot and mouth disease had not been seen since 1908, the virus arrived on straw imported from China. Another outbreak in South Africa began when a farmer picked up garbage from a tramp steamer anchored offshore and fed it to his pigs. The current British outbreak is thought to have begun in a similar way.

Because the disease is so devastating to farmers, countries go to great lengths to keep it out.

"The occurrence of even a single case of foot-and-mouth disease in a previously disease free country results in an immediate ban on an economically valuable export trade," the British Medical Journal noted in an editorial this month.

The U.S. Department of Agriculture's greatest fear is that a tourist returning from abroad will introduce foot-and-mouth disease to the United States by bringing home meat, unpasteurized dairy products or other food tainted with the virus. During a recent informal study in the Miami airport, Coats said, officials intercepted a half-ton of contraband food products in one hour.

Alternatively, hikers might transport foot-and-mouth disease from Yorkshire to New York on their boots. Authorities are so concerned that people who have recently visited the British countryside are being asked to disinfect their shoes before entering the United States.

Vaccines are available, but farmers are loath to use them. Like the annual flu shot for humans, they have to be tailored to a particular strain of the virus. And it is impossible to distinguish an inoculated animal from an infected one, because the vaccine produces the same immunological response as the disease. So the mere act of vaccinating a single cow or pig against foot-and-mouth disease can jeopardize a nation's meat and livestock exports.

Researchers at the USDA's Plum Island Animal Disease Center in New York State are working on an improved vaccine that would produce an antibody response distinguishable from the disease, but it will not be available in time to stem the current outbreak.

"We are literally years away from having it ready," said department spokeswoman Sandy Miller Hays.

The United States has not seen a case of foot and mouth since 1929, but it is common and widely feared in Asia, Africa, South America and parts of Europe.

People are almost never infected by the virus, and when they are it causes little or no illness. But in cattle, pigs and other hoofed animals, the disease is catastrophic. It causes such painful sores on the mouth, lips, feet and teats that animals are frequently unable to eat, drink or walk. Animals with the disease often lose so much weight that their value as meat or milk diminishes to nothing.