A virus is a microscopic organism consisting of genetic material (RNA or DNA) surrounded by a protein, lipid (fat), or glycoprotein coat. The name virus is derived from the Latin, means toxin or poison.
Some microbiologists classify viruses as microorganisms, while others don't because they are "nonliving" and describe viruses as microscopic infective agents. Viruses also lack the properties of living things: They have no energy metabolism, they do not grow, they produce no waste products, and they do not respond to stimuli. They also don't reproduce independently but must replicate by invading living cells.
Viruses, do, however, share a few features: First, they generally are quite small, with a diameter of less than 200 nanometers (nm). Second, they can replicate only within a host cell. Third, no known virus contains ribosomes, a necessary component of a cell's protein-making translational machinery.
Viruses are unique microorganisms because they cannot reproduce without a host cell. After contacting a host cell, a virus will insert genetic material into the host and take over that host's functions. The cell, now infected, continues to reproduce, but it reproduces more viral protein and genetic material instead of its usual products. It is this process that earns viruses the classification of "parasite".
It is quite interesting to know about friendly virus (bacteriophages) as friendly bacteria that exists in our intestines and help us digest food. They help to protect us from dangerous bacteria, like E. coli.
Viruses consist of strands of the genetic material nucleic acid, the basis of a genome, which is surrounded by a protein coat called a capsid. The capsid protects the genome and gives the virus its shape. Viruses may be either helical or icosahedral. Some viruses display a combination of helical and icosahedral symmetry, known as complex symmetry. The capsid is often subdivided into individual protein subunits called capsomeres. The organization of the capsomeres yields the symmetry of the virus. Animal viruses often form an envelope around the capsid. This envelope is rich in proteins, lipids, and glycoprotein molecules