C型肝炎病毒感染是本地慢性肝病的第二大原因，B肝表面抗原陰性的慢性肝炎息者中，70~80%為C肝抗體陽性，且多數體內存有病毒。一般健康成人的C肝抗體盛行率為1~2%，但隨年齡增加而上昇。台灣地區C型肝炎病毒感染以水平傳染為主要的傳染遼徑，其中因輸血而感染者約佔慢性C型肝炎的30~40%。垂直傳染或周產期傳染的效率不高，在10%以下，且和母體之高病毒濃度及生產方式有關。自民國81年7月進行捐血者C肝抗體篩檢後，輸血後急性C型肝炎的發生率已大幅降低，幾不復見。台灣地區最常見的C型肝炎病毒基因型為1b，其次為2a型和2b型。病毒濃度驟增，不同基因型病毒混合感染和宿主免疫反應可能和慢性C型肝炎之急性發作有關。C肝抗體陽性之肝硬化病人每年有3~5%的機會發生肝癌。治療上，單獨使用干擾素只有不到10%的持久反應，而合併療法（干擾素加Ribavirin）則有50%的持久反應率。

Hepatitis B virus (HBV) has long been known to be the major etiologic factor of chronic liver diseases and hepatocellular carcinoma (HCC), and in Taiwan 80-90% of chronic liver diseases and HCC are caused by HBV. Cloning of hepatitis C virus (HCV) genome and subsequent development of assays for antibodies against HCV have revealed HCV as the next most common cause of these diseases in Taiwan. The prevalence of antibodies against HCV (anti-HCV) in hepatitis B surface antigen (HBsAg)-negative patients is around 70-80%, and most of them are viremic. Anti-HCV is found in 1-2% of healthy adults, and increases in parallel with age. The epidemiology of HCV infection in Taiwan is similar to other areas of the world, with horizontal transmission as the major route of infection. Blood transfusion was an important route of transmission, accounting for 30-40% of chronic HCV infection. After screening for anti-HCV in blood donors was instituted in July 1992, this infection route was effectively controlled. In contrast, vertical or perinatal transmission plays a minimal role in the spread of HCV. The predominant genotype is type lb. being detected in 66-71% of patients with chronic hepatitis C and in 83% of those with cirrhosis or HCC, followed by type 2a (20%) and 2b (10%). Analysis of serum HCV cDNA levels showed that the levels ranged from 101 to 107 copies/ml and the serum virus levels were higher in patients with genotype 1b than those with type 2a or type 2b (p<0.005), indicating genotype as an important determinant of levels of HCV viremia. Abrupt increase of virus titer, mixed infections of multiple genotypes of HCV and host immune response may contribute to the acute exacerbations of chronic hepatitis C. The incidence of HCC in anti-HCV-positive cirrhotics was studied by a prospective follow-up study, and the results showed that 3-5% of these patients developed HCC each year. The mean age when HCC was detected was 63+9 years. The results indicate a high incidence of HCC in anti-HCV-positive cirrhotic patients in Taiwan. Regarding the treatment of chronic hepatitis C, sustained response (normalization of serum transaminases and clearance of HCV RNA after stopping therapy for 6 months) to interferon (IFN) alfa is unsatisfactory. Recently, our randomized controlled study has shown that combining ribavirin with IFN alfa induces a significantly higher sustained response than IFN alone in the treatment of chronic hepatitis C (43% vs. 6%).