This is the reason why it is very hard to manufacture antiviral drugs. It is difficult to make the antiviral drugs because, viruses use the host cell's to replicate. Thismake it difficult to find targets for the drug that would interfere with the virus without also harming the organism's cells.
Virus is a non-living substance when present in the external environment. Virus becomes active when it enters the host cell. As it has only genetic material and a protein coat,it has to depend on host cell machinery for replication. In this way it overtakes the host cell apparatus.
Anti viral drugs should target exactly the virus. But since viral particle lives inside the host and gets integrated into the host cell, application of antiviral therapy through drugs is very tough. Anti viral drugs may also kill the host cell also. This is the reason why it is very hard to manufacture antiviral drugs.