1. Life can breathe.
2. Life can grow.
3. Life can reproduce.
Of course, one of the flaws is that under this definition, a flame could be considered alive. I personally entertain the possibility that it could be a very, very simple and limited form of life. It has unusually complex behavior for something that isn't alive...
It's impossible to define life and have no grey areas.
^ This but for a general guideline:
Shared Characteristics of life
1) Reproduces by using DNA
2) Growth and Development
3) Actively take in energy from their environment (metabolize)
4) Organism must Sense and Respond to their environment
5) Maintain Homeostasis (constant internal conditions i.e. Human temperature can only fluctuate from 94 - 107 degrees before death occurs)
6) Can evolve as groups *never as an individual*
To answer your question about viruses according to this list they are missing some key characteristics of life.
1) Viruses lack the structure within cells that are necessary for life. They cannot on their own, partake in homeostasis, reproduction, or energy collection. Viruses hijack the hosts cells to preform these tasks that they cannot do on their own.
2) Viruses don't always reproduce via DNA some use RNA.
3)Viruses aren't actually cells, they are much simpler. They usually consist of a strand of heredity material (DNA/RNA) that is wrapped up in proteins.
"It is not length of life, but depth of life." ~ Ralph Waldo Emerson