Viruses are a essentially genetic information wrapped up in a protective protein layer. Viruses by their nature tend to be 'infective' agents. They are in essence basic behaviour replicators that require other living systems to replicate and survive. In a sense viruses can be parasitic or symbiotic.
Some viruses can lie dormant for long periods of time or go through short aggressive replication cycles that cause major disruptions in the health of an organism.