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’)