Hepatitis is any of several liver diseases characterised by inflammation, liver enlargement, jaundice, fever and abdominal pain. It can be caused by a number of different etiologies: some of these are drug, alcohol, or toxin-induced hepatitis, autoimmune disease, cholestasis, and viral hepatitis.

The commonest forms of viral hepatitis are known as hepatitis A, hepatitis B, and hepatitis C; all three of these are caused by viruses that can be transmitted sexually, by blood transfusion, or by shared syringes (see needle-exchange programme).

Hepatitis A (which is often a milder form of this disease), is frequently transmitted by contaminated food, a route called fecal-oral contamination.

Hepatitis B more often involves transmission by exposure to blood or other body fluids. About 1 million people die worldwide as a result of hepatitis B, often either of liver failure or liver cancer.

Two other viruses are known, hepatitis D and E, but considered as "additional" complications for types B and C.

Other viruses, such as cytomegalovirus and Epstein-Barr virus, can also cause infectious hepatitis.

Hepatitis C infection can exist undetected for periods as long as 10 to 20 years, and researchers estimate that millions of people are infected and have not yet displayed any symptoms.