Hepatitis B is caused by the hepatitis B virus (HBV). HBV is a noncytopathic virus. This means that the virus itself does not cause direct damage to liver cells. Instead, it is the immune system's aggressive response to the virus that usually leads to inflammation and damage to the liver (hepatitis). However, HBV can cause damage to the genetic material inside liver cells. This can lead to liver cancer which, like hepatitis, can also be fatal.
People who have not been infected with HBV can be vaccinated against the virus to prevent infection.
HBV is very similar to HIV in the ways it is transmitted: through direct blood-to-blood contact and through sexual activity. However, blood levels of HBV are much higher than for HIV or the hepatitis C virus, making this virus much easier to transmit in certain situations (e.g., from mother to child during delivery).
HBV is present in blood, semen, and vaginal fluids and is transmitted primarily through sexual activity. Another major transmission route is sharing injection drug equipment (including needles, cookers, tourniquets) and, to a lesser extent, non-injection drugs (cocaine straws and crack pipes) due to the possibility of exposure to blood. Pregnant women who have hepatitis B can also transmit the virus to their babies, most likely during birth.
The number of new hepatitis B infections in the U.S. has declined from about 260,000 a year in the 1980s to about 73,000 in 2003, with the greatest decline occurring in children and adolescents due to routine HBV vaccination.