Viruses are very primitive microbes. They have the unique ability to infect and parasitize living hosts, yet, they themselves are not alive. They do not ingest or process nutrients, they do not respirate, they do not excrete waste products and they are incapable of movement. Their sole reason for existence is to perpetuate and disseminate their own species.
A virus must rely on living living cells of suitable hosts for its reproduction. Outside of a cell, a virus is simply an inert bit of matter. Once taken inside a cell, the virus proceeds to take over the machinery of the cell, converting it into a factory for the construction and release of countless new virus particles.
Since most viruses destroy their host cells, they stimulate an immune response that not only attempts to destroy the invading virus particle and infected host cells, but can also cause serious and life threatening side effects. However, there are viruses that cause very little damage to the host cells and can remain dormant for years.
Fortunately, most virus infections are so mild that they go undetected. Unfortunately, very few diseases caused by viruses respond to specific antiviral medication. Usually, the care of an infected host is to treat the secondary effects of the infection - stabilizing the patient and protecting against secondary infection from bacteria or fungi.
The most effective therapy for any virus disease is prevention. The most common viral diseases in dogs have effective vaccines. Make sure your pet is vaccinated against all viral diseases that present a health hazard to your animal in your particular area.