The hepatitis B virus is usually transmitted via infected blood, sexual contact, or during birth. The hepatitis B virus is much more contagious than HIV (the virus that can lead to AIDS) or the hepatitis C virus. Hepatitis B virus is only transmitted from human to human.

Unlike hepatitis C, hepatitis B is commonly transmitted by sexual contact. People with detectable levels of virus in their blood should use condoms to protect their partners. Hepatitis B may also be transmitted in saliva and other body fluids. Therefore, vaccination of sexual partners is important.

The hepatitis B virus may be transmitted via blood or blood products. The modern test methods used to screen blood for use in transfusions are highly sensitive, and the risk of transmission by this route is very slight as a result. The virus may also be transmitted through sharing or reusing syringes or needles. Risk factors for contracting hepatitis B therefore include drug use, tattoos and body piercing. Transmission of the hepatitis B virus via open wounds, razor blades and toothbrushes is also possible.

The risk of infection in a newborn baby whose mother has hepatitis B is greatest during or shortly after birth. Some forms of chronic hepatitis B produce only small amounts of the virus in the body (low-replicative chronic hepatitis B). Other forms of the disease produce the virus in very large amounts (high-replicative chronic hepatitis B). The risk of virus transmission during delivery ranges from 10% (low-replicative chronic hepatitis B) to almost 100% (high-replicative chronic hepatitis B). Therefore, babies born to a mother with hepatitis B must always receive active and passive immunoprophylaxis immediately after birth (simultaneous vaccination and administration of immunoglobulin).

Opinions differ as to whether hepatitis B infection can be transmitted by breastfeeding. The likelihood of virus transmission during breastfeeding seems to be related to the mother's viral load.

A vaccine against hepatitis B is available.

Recommendations on hepatitis B vaccination vary between countries, according to the local epidemiology and available resources. In many countries infants, small children and adolescents are routinely vaccinated against hepatitis B. As a result the prevalence of hepatitis B has fallen greatly since the introduction of routine vaccination.

Other populations that may be recommended to have hepatitis B vaccination include people with a high risk of exposure through their work (medical and dental professions, the police force, first-aid workers), dialysis patients, all people with other chronic liver diseases (e.g. chronic hepatitis C), people living in close contact with people with chronic hepatitis B, and babies born to infected mothers. Hepatitis B vaccination is recommended for the sexual partners of people diagnosed with hepatitis B, for all gay and bisexual men, and for anyone diagnosed with HIV.

Three injections of the vaccine are required to ensure adequate protection, after which 90% of those vaccinated are protected from infection.