People with hemophilia lack substances (clotting factors) that permit blood to clot normally. They frequently need treatment with clotting factors that come from donated blood. Unfortunately, in the days before blood banks were able to routinely test donated blood for viruses, persons with hemophilia often got infections from the clotting factors. These infections included such viruses as human immunodeficiency virus (HIV, the virus that causes AIDS) and hepatitis viruses (a group of viruses that can cause liver problems). Hepatitis G is a virus that is commonly found in blood, but unlike hepatitis B and hepatitis C, it does not seem to cause any chronic liver problems. In fact, preliminary reports suggest that people who are infected with both HIV and hepatitis G may progress to AIDS more slowly than people who have only HIV infection.