“We are in the midst of an epidemic of fear,” said James Mason, director of the US Centers for Disease Control, in 1985. He was referring to the public's reaction to AIDS, then a newly emergent disease.

Fast-forward to 2001, and there is another epidemic of fear. Over the past few months, it has become clear that bovine spongiform encephalopathy (BSE) is a pan-European problem. Countries such as Germany, Italy and Spain, which for years regarded themselves as BSE-free, have been forced to admit that their cattle herds are harbouring the disease. Consumers feel betrayed, and sales of beef have plummeted across the continent. The shock wave is also being felt in the United States, where federal agencies are scrambling to prevent the disease crossing the Atlantic (see Nature 409, 441–442; 2001).

But there is one important difference between AIDS in 1985 and BSE in 2001. In March 1985, the US Food and Drug Administration approved the first blood test for HIV infection. At a stroke, it became possible to screen donated blood for the virus. For those lucky enough to live in countries with well-regulated blood supplies, avoiding AIDS became primarily a matter of abstaining from risky sexual practices.

By contrast, there is still no reliable way to identify cattle incubating BSE, or people infected with its human form, variant Creutzfeldt–Jakob disease (vCJD). In an attempt to put some figures to the BSE epidemic, the European Commission has instituted a programme of diagnostic testing of brain tissue from slaughtered cattle (see page 658). But while the tests being used may have some value in picking up preclinical cases, they have not been validated for this purpose. Unfortunately, the equivalent of the HIV blood test — a reliable diagnostic capable of identifying infected animals or people soon after they become infected — is not yet available.

For agencies confronting the BSE epidemic, producing such a test should be a top priority. Not only would it be a valuable epidemiological tool, but it would also offer a means to reliably exclude infected animals from the human food chain. And because potential therapies are likely to require intervention before symptoms appear, progress in diagnostics is also key to saving people who are incubating vCJD (see page 660).

But there is a danger that the validation of candidate diagnostic tests could become a bottleneck. It will require the tests to be put through their paces on tissue samples taken from animals that were deliberately infected but have yet to show any symptoms. Currently, the only source of such material is Britain's Central Veterinary Laboratory in Weybridge, Surrey. To ensure that sufficient reference material is available, the European Commission should move urgently to create a central repository.