On its own, a virus may be considered as an inert biochemical complex since it cannot replicate outside of a living cell. Because viruses are non-motile, they are entirely dependent on external physical factors for chance movement and spread to infect other susceptible cells.
Some proteins are non-structural, eg.
All the viral proteins have reactive epitopes which are important for interaction with cellular components during the process of infection and replication. View electron micrograph images of various viruses. Retroviruses are a special category of RNA viruses that require reverse transcription of their RNA to DNA and then integration of that DNA into the host cell genome before replication can take place. They carry a reverse transcriptase enzyme as part of the virion. Adsorption Viruses have reactive sites on their surface which interact with specific receptors on suitable host cells.