Viruses consist of genetic material (DNA, positive RNA or negative RNA) packaged into a nucleocapsid and/or membrane. Unlike other infectious agents, viruses are so small that they depend on the host cell machinery for protein synthesis, energy and replication.
Classification of viruses is based on their genetic material, envelope and/or nucleocapsid, size and morphology. Treatment of viral infections is aimed at the inhibition of the transcription and replication.
A nucleocapsid and a membrane with glycoproteins characterize an enveloped virus.
Viruses containing RNA genomes must also encode the RNA replicases in order to survive.
Extra info: Without attracting the host enzymes to replicate their particular genome, viruses would not survive.