Robust HCV Infection in vitro Five days after these cells were inoculated with a tiny amount of hepatitis C virus (one infectious virus particle for every 100 cells), the infection has spread. Staining with a fluorescent antibody that targets the viral protein NS5A reveals that virtually all of the cells are infected with the virus, which shows that there was an approximately 100-fold in vitro expansion of the virus in just 5 days. Image by Jin Zhong.