Hepatitis B is a serious liver disease, caused by the hepatitis B virus (HBV). The virus is transmitted by direct blood-to-blood contact, during unprotected sex, or by using unsterile needles. An woman with HBV can infect her newborn during the delivery process. There is a simple blood test to determine if you have been infected, and a safe and effective vaccine. New treatments are available for those who have chronic infections.