They can Interfere with viral particle attachment to host cell membranes or uncoating of viral nucleic acids Inhibit a cellular receptor or factor required for viral replication Block specific virus-encoded enzymes and proteins that are produced in the host cells and that are essential for viral replication but not for normal host cell metabolism Antiviral drugs are most often used therapeutically or prophylactically against herpesviruses (including cytomegalovirus), respiratory viruses, HIV, chronic hepatitis B, and chronic hepatitis C.