Hepatitis B is a disease of the liver caused by the Hepatitis B virus (HBV), a member of the Hepadnavirus family[1] and one of several unrelated viral species which cause viral hepatitis. It was originally known as "serum hepatitis" and has caused current epidemics in parts of Asia and Africa.[2] Hepatitis B is recognized as endemic in China and various other parts of Asia.[3] The proportion of the world's population currently infected with the virus is 3 to 6%, but up to a third have been exposed. Symptoms of the acute illness caused by the virus include liver inflammation, vomiting, jaundice, and rarely, death. Chronic hepatitis B may cause liver cirrhosis which may then lead to liver cancer. The hepatitis B virus is the second most prevalent cause of cancer in humans after Tobacco smoke.
Virions consist of an outer lipid envelope and an icosahedral nucleocapsid core, the latter being composed of both protein and DNA. The outer envelope contains embedded proteins which are involved in viral binding of, and release into, susceptible cells. Virion shape is generally spherical with a diameter of 40 - 48 nanometers (nm) but pleomorphic forms exist, including filamentous and spherical bodies lacking a core. These "subviral" particles are not infectious.
The DNA genome is not segmented but rather partially double-stranded, containing a long and short segment which overlap approximately 240 nucleotides to form an open circle. The longer strand is 3020-3320 nucleotides long, and the shorter is 1700-2800 nucleotides long.[4] The virus can be divided into four major serotypes (adr, adw, ayr, ayw) based on antigenic epitopes present on its envelope proteins, and into eight genotypes (A-H) according to overall nucleotide sequence variation of the genome. Different genotypes have distinct geographic distributions. For example, genotypes B and C are prevalent in China and neighboring countries.
