The true risk of developing either CJD or vCJD is largely unknown. In February 2004, scientists in Italy discovered a new form of mad cow disease. The brains of the infected animals resembled those of people with CJD, rather than those of cattle with BSE, leading to speculation that undetected prion diseases might be responsible for some apparently spontaneous cases of CJD. The Italian discovery emphasizes how little is known about prion diseases and how uncertain the ultimate risk of exposure may be.

At the same time, scientists have identified certain factors that increase your risk of CJD. They include:

• A family or genetic history of CJD. A few people have a genetic mutation that increases their likelihood of developing CJD.

• Human growth hormone injections. You're at increased risk of CJD if you have received HGH from human sources. This is most likely to have occurred before a genetically engineered form of the hormone became available in the mid-1980s.

• Certain medical procedures. Worldwide, a small number of people have developed CJD from infected tissue used in cornea and skin transplants and from dura mater grafts used during brain or spinal cord surgery.

• Contaminated surgical instruments. A few people have contracted CDJ from contaminated instruments used during brain surgery.

If you eat beef from countries with a relatively high incidence of BSE — such as the United Kingdom, Portugal, France, Spain and Germany — you increase your chances of contracting vCJD. The risk is highest if you eat meat products that are more likely to contain prions, such as hamburgers, hot dogs, sausages and deli meats. Even so, in the UK the current risk of acquiring vCJD from beef and beef products appears to be very low — about one case per 10 billion servings. The risk from beef in other high-incidence countries is also small.

On the other hand, it's difficult to assess the true risk of contracting vCJD from infected beef because so much about the disease is still unknown, including the exact length of its incubation period and the ultimate effectiveness of public health control measures. People believed to be at greatest risk are those who ate British beef between 1980 and 1996, before measures to prevent the spread of mad cow disease were implemented. The risk in other countries where BSE is endemic remains unclear.

You also may be at risk if you received a blood transfusion or used bovine-derived insulin after 1980 in a country with a high incidence of mad cow disease, such as Great Britain, Portugal, Spain, France and Germany.