My husband and I are thinking about adopting a baby. He is not born yet, and his mother is a carrier of hepatitis C. What are the chances of the child being born a carrier or with the disease? What does being a carrier mean?

J.M.

First, the term "carrier" means that one is chronically infected with an organism yet has no illness. A classic example of an infectious carrier was Typhoid Mary. She was a carrier of Salmonella typhi, the bacterium that causes typhoid fever. She was not ill with the infection, but she was able to spread the organism to others, many of whom became ill.

Hepatitis C is an infection of the liver caused by the virus of the same name. (Hepatitis means "liver inflammation.") Most people who become infected with the hepatitis C virus do not clear it from their system. They develop a chronic infection. Some will have symptoms such as fatigue, general ill feeling and low-grade fevers, but most patients have no symptoms at all. After 20 years, probably between 20 percent and 40 percent of people with chronic hepatitis will develop permanent, severe liver damage called cirrhosis. Patients who have no symptoms and who never develop any consequences of hepatitis C infection could be considered carriers of the virus. But some of those who have no symptoms are still experiencing active liver damage. A liver biopsy is needed to identify these patients.

The risk of an infected mother passing hepatitis C to her baby (whether she feels ill or not) is low, lower than the risk of transmitting HIV or hepatitis B. For hepatitis C, the overall risk of transmission is about 5 percent, though some studies on this topic have shown conflicting results. A few factors may increase the risk slightly. The first is a high viral load in the mother. This means that the more virus she has, the more likely she is to pass the infection to her infant. Another factor is HIV. Mothers who are infected with both HIV and hepatitis C are more likely than those without HIV to pass hepatitis C to their children. One study showed an increase in the risk of transmission after vaginal delivery as compared to Cesarean section. This was not seen in other studies.

Overall, a 5 percent risk of transmission seems quite low. If you are concerned, you could ask for the mother to have a hepatitis C viral load test. If her viral load is low (less than 10,000) or undetectable, the risk would be very low.

One other point is that it is a little difficult to diagnose hepatitis C in a newborn. A test of disease-fighting antibodies in the blood, the usual way to diagnose the infection and the way it was probably diagnosed in the mother, is unreliable in newborns. (That is because a mother's antibodies may cross the placenta and circulate for a time in the newborn's bloodstream.) The only reliable way to make the diagnosis in a newborn is with repeated tests called PCR. These tests look for the actual genetic material of the virus in the patient being tested.