Hepatitis-B is a serious infection on the liver which is caused by the hepatitis B-virus. You can catch it through unprotected sex or through blood to blood contact with someone who has it. It can be passed on by sharing needles or other injecting equipment, you can also be infected by sharing toothbrushes, razors, sex toys and anything that may have blood on it. It is a virus because it can only multiply within another cell of a host.