The strain of foot-and-mouth virus plaguing Britain's farms was first detected in India more than a decade ago. Scientists have been tracking it across the world since then, but are no closer to determining how it got to England.

Considered the most contagious disease in the world, foot-and-mouth is widespread in Africa, the Middle East, Asia and South America. It affects cloven-hoofed farm animals and can wreck a country's livestock export business, though it is harmless to humans.

The virus can be spread by anything it touches, from sandwich meat to the soles of shoes and truck tires; wind can carry it 40 miles.

While countries in the developed world scramble to stamp it out as soon as it is detected to protect trade, poor nations that don't export meat let it run its course.

Experts have identified the virus causing the current outbreak in Europe as belonging to the Pan-Asia type 0 strain. The subtype ravaging Britain is normally found in the Middle East and South Asia.

"In some cases, you could track the exact movements of the virus," said Chris Bostock, director of the Institute of Animal Health in Pirbright, England.

"But with this strain of type 0, it has cropped up in so many places around the world that you can't really say this led to this, and then it went there ... It could have gotten here from any of the places it has been," he said.

There are seven main varieties of the foot-and-mouth virus and several subtypes of each. Scientists use genetic tests to determine which subtype is causing a particular outbreak.

All outbreaks are reported to the Institute for Animal Health, the world reference laboratory for foot-and-mouth disease, which tracks occurrences across the globe.

Scientists then examine samples of infected animals and trace the genetic lineage in hopes of finding the source of the infection.

The strain of virus ravaging Britain was first identified in 1990 in northern India.

"That doesn't mean to say it wasn't causing problems elsewhere at an earlier time," Bostock said. "That is just the first time it was detected."

In 1993, it was found to the north in neighboring Nepal and by 1994 it had spread westward to Saudi Arabia. In 1996, it caused outbreaks throughout the Middle East and moved into the western fringe of Europe, touching parts of Turkey, Bulgaria and Greece. At the same time it cropped up in Malaysia and Bangladesh.

By 1999, it was detected in mainland China, then Taiwan. In late 1999 and in 2000, it reached most of Southeast Asia.

Last year, it was reported in Japan and Korea and in Mongolia and remote parts of eastern Russia.

According to the World Organization for Animal Health, circumstantial evidence indicates hay imported from China may have been a factor in the simultaneous outbreaks in Korea and Japan.

The agency says the outbreaks in Taipei in 1999 and in Mongolia in 2000 were probably due to the illegal movement of live infected animals from neighboring countries, and the outbreak in eastern Russia was probably caused by feeding pigs with infected products illegally imported from China or other countries.

The first case in Britain was confirmed Feb. 20 in pigs in a slaughterhouse in northern England.

By the time the disease was confirmed, infection had been transmitted through markets, primarily by sheep, experts said.

Scientists don't know how the virus got to England. Agriculture Minister Nick Brown suggested earlier this week that it may have arrived on an illegal meat shipment which got into the swill fed to pigs at Heddon-on-the Wall in northern England, believed to be the starting point of the epidemic.

"I suspect it will be very difficult to find out how it got here. It's basically too late to trace it," Bostock said.