The donor ban is strictly a precaution -- there is no evidence that any mad cow-type illness has been spread through blood transfusions.
But the mad cow disease that swept through Britain's cattle has been linked to a human brain-destroying illness, and both illnesses are so mysterious that scientists simply can't rule out the possibility they could infect blood.
The FDA has told blood donation centers in the United States to stop accepting blood from anybody who spent a total of six months in the United Kingdom, meaning England, Scotland, Wales, Northern Ireland, the Isle of Man and the Channel Islands.
The average tourist who spent just a few weeks in Britain can still donate blood, but people who went to Britain repeatedly between 1980 and 1997, the crisis years, will have to add up their trips to see if they're under the six-month limit. Canada issued a similar restriction Tuesday, and Australia and Japan said they were considering doing the same.
Still, the FDA's donor ban is controversial. Health experts fear it may frighten Americans whose blood is refused.
Worse, the American Red Cross estimates the ban will cut U.S. blood donations by 2.2 percent at a critical time. While that doesn't seem like a large percentage, it translates into 285,00 units of blood.
Even before Tuesday's action, experts were predicting severe, nationwide blood shortages to hit as early as next year because blood donations already were falling by about 6 percent in the last few years. Meanwhile, blood transfusions have risen about 4 percent.
"We certainly do not intend by this to scare people," said Mary Elizabeth Jacobs, the FDA scientist who coordinated the policy's development.
The FDA plans to work with blood banks to find ways to ease the donor loss. But James MacPherson of America's Blood Centers -- which represents blood banks that provide half the nation's supply -- said the government also needs to help blood banks explain the issue to the confused or frightened donors they turn away.
"It's going to be an extremely difficult issue for people to understand," he said.
At issue is an infection that kills by literally eating holes in brain tissue. In cattle, it's called mad cow disease, and it swept through British herds starting in the late 1980s.
About one in 1 million people around the world gets a similar brain disease called Creutzfeldt-Jakob disease, or CJD. Although CJD sometimes is hereditary, usually its cause is not known.
The worry about blood stems from Britain's dscovery in the mid-1990s that some people apparently caught a new strain of CJD by eating beef infected with mad cow disease. Named "new variant CJD," it has killed 41 Britons.
There is no known mad cow disease in U.S. cattle. The United States has not allowed importation of British beef for over a decade and no American is known to have caught new variant CJD.
Much is not known about the new human disease, including exactly how Britons were sickened. Theoretically, even a brief visit to Britain could have exposed someone to new variant CJD. But the FDA settled on the six-month limit to reduce the possible risk without slashing the U.S. blood supply; banning donors who spent less time in Britain would have had a far greater impact.
[For more information related to this story, see Spotting A Fatal Brain Disease.]