Let’s first define life. According to the online Merriam-Webster Dictionary, life is “an organismic state characterized by capacity for metabolism, growth, reaction to stimuli, and reproduction.”
Viruses are not living things. Viruses are complicated assemblies of molecules, including proteins, nucleic acids, lipids, and carbohydrates, but on their own they can do nothing until they enter a living cell. Without cells, viruses would not be able to multiply. Therefore, viruses are not living things.
When a virus encounters a cell, a series of chemical reactions occur that lead to the production of new viruses. These steps are completely passive, that is, they are predefined by the nature of the molecules that comprise the virus particle. Viruses don’t actually ‘do’ anything. Often scientists and non-scientists alike ascribe actions to viruses such as employing, displaying, destroying, evading, exploiting, and so on. These terms are incorrect because viruses are passive, completely at the mercy of their environment.
Update: See a more recent post for my thoughts on this question.