The virus is transmitted by exposure to infectious blood or body fluids such as semen and vaginal fluids, while viral DNA has been detected in the saliva, tears, and urine of chronic carriers. Perinatal infection is a major route of infection in endemic (mainly developing) countries. Other risk factors for developing HBV infection include working in a healthcare setting, transfusions, and dialysis, acupuncture, tattooing, extended overseas travel and residence in an institution. However, Hepatitis B viruses cannot be spread by holding hands, sharing eating utensils or drinking glasses, kissing, hugging, coughing, sneezing, or breastfeeding.

The acute illness causes liver inflammation, vomiting, jaundice, and (rarely) death. Chronic hepatitis B may eventually cause cirrhosis and liver cancer—a disease with poor response to all but a few current therapies. The infection is preventable by vaccination.

Hepatitis B virus is an hepadnavirus—hepa from hepatotropic (attracted to the liver) and dna because it is a DNA virus—and it has a circular genome of partially double-stranded DNA. The viruses replicate through an RNA intermediate form by reverse transcription, which practice relates them to retroviruses. Although replication takes place in the liver, the virus spreads to the blood where viral proteins and antibodies against them are found in infected people.