Hepatitis B is a liver disease that results from infection with the Hepatitis B virus. The virus has a small circular DNA genome that is partially double-stranded and it is member of the family Hepadnaviridae. It can range in severity from a mild illness lasting a few weeks to a serious, lifelong illness. Hepatitis B is usually spread when semen, blood, or another body fluid from a person infected with the Hepatitis B virus enters the body of someone who is not infected. Hepatitis B can also be passed from an infected mother to her baby at birth.