Though they vary as science evolves, the basic requirements for life are that it must:
* Obtain and use energy
* Grow, develop, and die
* Respond to the environment
Arguably, a virus does all or none of these, or simply some. Though they reproduce, they cannot do so without another living cell--they are purely parasidic. However, they do have their own DNA, which evolves/mutates over generations as it meets resistance [environmental response?], though one unit in itself shows no developmental stages except for the infusion of its data into another cell, inside of which it uses the cell's resources to multiply.
Scientists have been going crazy on this matter for years... can religious philosophy provide a perspective here?