HBsAg is the hepatitis B surface antigen. Whether or not a positive hepatitis B surface antigen is contagious is based primarily on the hepatitis B virus HBV-DNA. If the hepatitis B virus is HBV-DNA positive, it is usually contagious. Hepatitis B surface antigen itself is the outer shell protein of the hepatitis B virus and is only antigenic, not infectious. The main routes of infection for hepatitis B are through the blood route, mother-to-child transmission and sexual transmission. If you share syringes or personal items such as toothbrushes, nail clippers, razors, and shavers with someone with hepatitis B, you are at risk of contracting the hepatitis B virus. If an adolescent child has hepatitis B, most are due to vertical transmission of infection from mother to child.