Mad cow disease, or bovine spongiform encephalopathy (BSE), is a disease that was first found in cattle. It's related to a disease in humans called variant Creutzfeldt-Jakob disease (vCJD). Both disorders are universally fatal brain diseases caused by a prion. A prion is a protein particle that lacks DNA (nucleic acid). It's believed to be the cause of various infectious diseases of the nervous system. Eating infected cattle products, including beef, can cause a human to develop vCJD.

Mad cow disease is a progressive, fatal neurological disorder of cattle resulting from infection by a prion. It appears to be caused by contaminated cattle feed that contains the prion agent. Most mad cow disease has happened in cattle in the United Kingdom (U.K.), and a few cases were found in cattle in the U.S. between 2003 and 2006. Feed regulations were then tightened.

In addition to the cases of mad cow reported in the U.K. (78% of all cases were reported there) and the U.S., cases have also been reported in other countries, including France, Spain, the Netherlands, Portugal, Ireland, Italy, Japan, Saudi Arabia, and Canada. Public health control measures have been implemented in many countries to prevent potentially infected tissues from entering the human food chain. These preventative measures appear to have been effective.

Creutzfeldt-Jakob Disease (CJD) is a rare, fatal brain disorder. It causes a rapid, progressive dementia (deterioration of mental functions), as well as associated neuromuscular disturbances. The disease, which in some ways resembles mad cow disease, traditionally has affected people between the ages of 50 and 75. The variant form, vCJD, however, affects younger people (the average age of onset is 28) and has observed features that are not typical as compared with CJD. About 230 people with vCJD have been identified since 1996. Most are from the U.K. and other countries in Europe. It's rare in the U.S.

Currently, this risk appears to be very small, perhaps fewer than 1 case per 10 billion servings, if the risk exists at all. Travelers to Europe who are concerned about reducing any risk of exposure can avoid beef and beef products altogether, or can select beef or beef products such as solid pieces of muscle meat as opposed to ground beef and sausages. Solid pieces of beef are less likely to be contaminated with tissues that may hide the mad cow agent. Milk and milk products are not believed to transmit the mad cow agent. You can't get vCJD or CJD by direct contact with a person who has the disease. A few cases were acquired during the transfusion of blood from an infected donor. Most human Creutzfeldt-Jakob disease is not vCJD and is not related to beef consumption but is also likely due to prion proteins.