Hepatitis C is an infectious disease affecting mainly the liver, caused by hepatitis C virus (HCV), an RNA virus of the Flaviviridae family which only infects humans and chimpanzees. A "non-A non-B hepatitis" was postulated in the 1970s, and HCV was demonstrated in 1989. HCV is spread primarily by blood-to-blood contact associated with intravenous drug use in the developed world, and with improperly sterilised medical equipment and blood transfusions in the developing world. In about 80% of those infected, the virus establishes a chronic infection in the liver, and around 10–30% of those infected will develop cirrhosis over 30 years. Some people with cirrhosis go on to develop liver failure, liver cancer or other serious complications.
An estimated 143 million people worldwide (2%) have chronic HCV infections as of 2015. The prevalence is highest in Central and East Asia, North Africa and the Middle East. The virus causes around a quarter of cases of cirrhosis and hepatocellular carcinoma, and is a major reason for liver transplantation. The recommended therapy is an appropriate combination of protease inhibitors. Although 95% of people treated in this way are cured, the treatments are expensive and older therapies are less effective. No vaccine against hepatitis C is available.