Why are most viruses not lethal?

Answer 1

While I have not seen numbers indicating that most viral infections are not lethal, it seems a reasonable assertion. (please don't consider my answer authoratative since I am drawing upon general knowledge of the Immune System and virology rather than specific research on the question)

The primary method of cellular anti-viral action is the interferon response. Viruses generally display counter-measures to the induction of this response. However, they are imperfect and invariably some viral derived material that can induce the response "slips by". As this material accumulates, the interferon response is induced and anti-viral activity is commenced which includes stoppage of all RNA production (stop viral transcription and replication), apoptosis (the infected cell kills itself), and the signalling of neighboring cells to enter an antiviral state.

So, it is essentially a race between the virus' ability to replicated and infect other cells, and the induction of the Interferon Response. Given that most viral infections are not lethal, one can conclude that the Interferon Response wins the race most often.

However, one most also consider the Adaptive Immune response which also clears viral infections. It's onset is generally observed to start 96 hours post infection. (leading one to wonder if its the antibiotics we just took or the adaptive immune response that clears our bacterial infections so quickly ;) )