Hepatitis B is an infection of the liver caused by the hepatitis B virus (HBV). Hepatitis B virus infection may occur in two phases. The acute phase occurs just after a person becomes infected, and can last from a few weeks to several months. Acute hepatitis is clearly "symptomatic". The individual suffers visibly and on recovery usually clears the virus from his/her system. Most people recover after the acute phase, but others, particularly those infected whilst young, remain infected for the rest of their lives. They go into the chronic phase, in an assymtomatic condition, and become "chronic carriers." The virus remains in their liver and blood and increases the likelihood of liver disease in later life. Many chronic carriers may not know they are infective. They may not even have become ill at the time of their infection.
Because of the virus' ability to survive on objects for a week or more, household contacts and childhood playmates of infected persons are at high risk. Hepatitis B virus and the HIV virus are transmitted in an identical way -- by blood contact -- but hepatitis B is 100 times more infectious than AIDS. Those people infected with HBV who become "chronic carriers" can spread the infection to others throughout their lifetime. They can also develop long-term liver disease such as cirrhosis (which destroys the liver) or liver cancer. Of the reported cases, approximately 1.4% end in death.
More than one third of the world's population has been infected with hepatitis B. Most recover, but 360 million carriers remain. One million people a year die from its complications, usually of liver cancer. Worldwide there are an estimated 300 million persons with ongoing hepatitis B infection. A significant percentage were infected in their early years of life -- whether from birth or at a very young age -- and are likely to die earlier or have a shortened life as a result. In developed areas, such as the United States, Western Europe and Australia, only 0.2 - 0.9% of the population is chronically infected. The prevalence of carriers in northern Europe is 0.1% or less, in central and eastern Europe up to 5%, and in southern Europe yet higher still. There is a significantly greater risk of HBV infection in developing areas, such as China and Southeast Asia, most of Africa, most Pacific islands, Haiti and the Dominican Republic, parts of the Middle East, and in the Amazon Basin. In these areas, approximately 8 - 15% of the population is chronically infected. In other parts of the world, there is a moderate risk of HBV infection, with 2 - 7% of the population being HBV carriers.
The highest prevalence is in the 20-40 year age group. Transmission from hepatitis B carrier mothers to their babies appears to be the single most important factor for the high prevalence in some areas. The degree of risk depends on the proportion of mothers who are carriers, which may be as high as 40% in some countries.