My daughter just received notice from the Red Cross that they destroyed her blood donation due to hepatitis C antibodies. She has never used drugs or had sex. How could she have gotten this?

M.L.C.

Hepatitis C is a viral infection that affects the liver. It is quite common, with about 3 million people infected in the United States alone. It appears that about 20 to 50 percent of patients with hepatitis C will develop severe liver disease after 20 years of infection.

In the past, blood transfusion was the most common method of transmission of hepatitis C in the United States. Now all blood is tested, and the risk of infection by transfusion is very low. Infection is most likely to occur through needles or other devices that allow percutaneous ("through the skin") exposure to the virus. Injecting illegal drugs is a common type of percutaneous exposure. Others occur with accidental needlestick injuries in health-care workers.

Hepatitis C can be acquired sexually, but the risk of this means of infection is relatively low. If a person with hepatitis C is in a stable, monogamous sexual relationship, the risk that the partner will become infected is about 5 percent. The U.S. Public Health Service does not recommend barrier precautions (such as condoms) for all hepatitis C patients in a stable monogamous relationship, though condoms probably do lower the chance of infection somewhat.

Another way that one can acquire hepatitis C is around the time of birth. Pregnant women infected with hepatitis C pass the infection to their babies 5 percent of the time. Household, nonintimate contact is not likely to result in transmission of hepatitis C. There are case reports, however, of rare times when this may have occurred.

Why did your daughter test positive? There are a few possibilities. First, there may have been a mistake. Not all of the tests used to screen blood for the virus are 100 percent accurate. I would repeat the test with a highly accurate test. If she indeed has hepatitis C, she may have acquired it through sex (despite what she told you) or after a percutaneous exposure that she does not recall. It is also possible that she was infected at birth, because most patients are without symptoms for quite a long time after infection. If she has ever donated blood before and her blood tested negative, that would rule out this possibility. There are also a substantial number of people who are infected with hepatitis C, perhaps 10 percent, in whom the means of acquisition can never be determined.