Why are viruses at the borderline of living and non-living things?

A virus has a very simple composition. It has a protein sheath inside which there is a strand of DNA. A virus shows some properties akin to living organisms. However, unlike other organisms, they show some properties that are akin to non-living things. They can undergo crystallization and in that form, survive for billions of years. They can survive very high temperatures, freezing cold and ultra-violet radiation in space vacuum, in crystallized form.