Introduction
Virus is an obligate intracellular parasite with a non-cellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small to be seen with the light microscope and thus must be studied by electron microscopes. In one stage of their life cycle, in which they are free and infectious, virus particles do not carry out the functions of living cells, such as respiration and growth; in the other stage, however, viruses enter living plant, animal, or bacterial cells and make use of the host cell’s chemical energy and its protein- and nucleic acid–synthesizing ability to replicate themselves. They lie somewhere in the grey area between living and non-living states.
