Antiviral drug was firstly reported regarding thiosemicarbazones developed for poxviruses in 1951. Then, human beings have been developed antiviral drugs using the latest technolo- gies including drug-design by computer, selection from huge compounds, antibodies and ge- nome editing. However, drug-resistant viruses always emerge because viruses are able to easily mutate. Although Favipiravir, which is a broad-spectrum antiviral drug, became a hot topic by having antiviral activity to several viruses including ebolavirus, viral drug-resistance would have been appeared by mutation in chikungunya virus. In this paper, the herpesvirus, HIV and influenza virus were focused to introduce development of antiviral drugs and the mechanism for emergence of drug-resistance.