Most cases of chronic viral hepatitis are caused by hepatitis B or C virus. Hepatitis B virus is one of the commonest chronic viral infections in the world, with about 170 million people chronically infected worldwide. In developed countries it is relatively uncommon, with a prevalence of 1 per 550 population in the United Kingdom and United States.

The main method of spread in areas of high endemicity is vertical transmission from carrier mother to child, and this may account for 40-50% of all hepatitis B infections in such areas. Vertical transmission is highly efficient; more than 95% of children born to infected mothers become infected and develop chronic viral infection. In low endemicity countries, the virus is mainly spread by sexual or blood contact among people at high risk, including intravenous drug users, patients receiving haemodialysis, homosexual men, and people living in institutions, especially those with learning disabilities. These high risk groups are much less likely to develop chronic viral infection (5-10%). Men are more likely then women to develop chronic infection, although the reasons for this are unclear.

Up to 300 million people have chronic hepatitis C infection mainly worldwide. Unlike hepatitis B virus, hepatitis C infection is not mainly confined to the developing world, with 0.3% to 0.7% of the United Kingdom population infected. The virus is spread almost exclusively by blood contact. About 15% of infected patients in Northern Europe have a history of blood transfusion and about 70% have used intravenous drugs. Sexual transmission does occur, but is unusual; less than 5% of long term sexual partners become infected. Vertical transmission is also unusual.