An ongoing question in virology is whether viruses are to be considered living creatures.  Its easy to tell that a groundhog is alive but a book is not.  But what properties does a groundhog have that a book does not?  We can look up basic properties of living things in a biology textbook, and yet it remains difficult to define life in a simple sentence.

I would argue that a virus is not alive. Viruses are completely dependent on host cells to replicate.  That said, in absence of the host cell the virus clearly lacks most of the properties of a living thing.  (Does stealing those properties from a living thing count towards being alive?)  Life seems to emerge from a collection of parts where the whole is greater  than the sum of the parts.  This emergent property, life, is present in animals, plants, bacteria etc, but in a virus infected cell, that property remains a part of the cell, not the virus.

Alive or not, viruses are an integral part of biology.  They help us understand life and they certainly have an effect on living things.