Because they are unable to accomplish life's processes by themselves, viruses are not considered living organisms. They are able to metabolize and reproduce only when they are within living cells. Thus, all viruses are parasites, and many of them cause disease.
Much smaller than the smallest bacteria, most viruses consist only of a strand or two of a nucleic acid – DNA or RNA - wrapped in a protein coat. Some also have a lipid envelope outside the coat. A virus reproduces and spreads because once its nucleic acid is inside a cell, the virus uses the cell's own DNA to produce additional copies of itself. (‘The New York Times Guide to Essential Knowledge’)