The discovery of hepatitis C was the direct result of the landmark discoveries of hepatitis B virus (HBV) and hepatitis A virus (HAV) and their serologies. Screening tests for HAV and HBV made it possible in the mid-1970s to examine cases of transfusion-associated hepatitis (TAH) and to demonstrate that only approximately 25% resulted from HBV and that none were related to HAV. Consequently, approximately 75% of TAH became classified as non-A, non-B hepatitis (NANBH). Subsequently, chimpanzee studies demonstrated that NANBH was a result of a transmissible agent. Although it has been difficult to convince clinicians that NANBH was a serious disease because the overt manifestations are generally mild, it gradually became apparent that the NANBH agent often resulted in chronic hepatitis and sometimes evolved into cirrhosis. The NANBH agent remained a virologic enigma for the next decade until researchers at the Chiron Corporation used an ambitious molecular approach on large volumes of high-titer infectious chimpanzee plasma from the Centers for Disease Control and Prevention (CDC). They extracted RNA, cloned it into an expression vector, and screened the expressed product with presumed immune sera. A single positive clone was found in the millions screened, and, within a year, the entire genome was sequenced and the agent was identified as a novel flavivirus—the hepatitis C virus (HCV). Retrospective analysis of pedigreed samples at the National Institute of Health (NIH) showed that 70% to 90% of NANBH cases were HCV related. The impact of HCV blood donor screening has been enormous. The single-antigen first-generation enzyme immunoassay (EIA-1) prevented 40,000 HCV infections within the first year, and the second-generation assay (EIA-2) has actually reduced new transfusion-related HCV infections to almost zero.