The failure of vaccines to induce a humoral response is an important reason why vaccine-preventable illnesses occur. Childhood lead exposure may be contributory reason for this vaccine failure. Research indicates that lead exposure inhibits the development of antibodies in laboratory animals. But we do not know whether this also occurs in people. The purpose of this study is to look into whether childhood lead exposure affects the humoral response after vaccination. To determine this, we will compare children who had lead poisoning when younger to children who did not. All eligible children will receive the human papilloma virus (HPV) vaccine (known as Gardasil). We will give this recommended vaccine and draw blood afterward to see if the child makes antibodies to HPV. A total of four appointments will be required (a first appointment, and then again at one, two and six months later). The vaccine will be given at the first appointment, and at two and six months after. A small blood sample will be drawn at each of the four appointments.