Viruses are minute, obligate intracellular parasites visible only through an electron microscope. Unlike bacteria, viruses have no metabolic activity of their own. Therefore, they depend on the cell of a living host for the required nutrients needed for metabolism and replication.
The structure of the virus is orderly and uniform. Its most important substance is a compound of protein and nucleic acid called nucleoprotein. The nucleic acid, either DNA or RNA, but not both, contain all the instructions that permit the virus to control the metabolic activity of the cell it infects. The DNA or RNA of the virus may be double-stranded or single-stranded. This nucleoprotein of the virus may be surrounded by one or more protein "coats." This protein coat contains enzymes and is referred to as a capsid and can have a variety of shapes from simple to complex structures. The capsid of some viruses may be surrounded by a lipid membrane called an envelope.
Viruses have the ability of being able to infect the living cells of a human body, animal, plant, or bacteria. They do this by injecting nucleoprotein into a cell and controlling the cell's metabolic mechanisms. The cell, in turn, makes the vital structures that become complete viruses. The host cell then bursts and dies, at which time it releases many more viruses to invade other cells. In addition viruses may produce toxins or act as antigens: substances the body recognizes as being foreign that it combats by producing antibodies. Viruses may remain inactive for a period of time before taking control of cellular metabolism and are the agents of many infectious diseases both acute and chronic. The term "virulence" is used to describe their ability to cause disease.
There are several ways viruses may be classified or grouped. They may be grouped by whether they contain DNA or RNA, or by type of host such as human, animal, plant, or bacteria. They may be classified by origin, mode of transportation, manifestation produced in the host, or by the geographical site where they were first located.
Treating viruses with standard drugs such as antibiotics is difficult for several reasons. Firstly, viruses have the ability to alter their individual characteristics and can continue to grow and replicate in new environments. They often survive the initial doses by changing their characteristics so that they rapidly become resistant to the drugs. Secondly, since viruses require the metabolic activity of the host cell, there is a danger of damaging or killing the host cell when trying to reach the virus. Therefore, viral treatment has mainly relied on defense by the host's immune system.
Viruses of rats by themselves have a low mortality rate within a laboratory setting. However, in an unprotected environment certain viruses can affect the immune system allowing opportunistic bacteria to become pathogenic. Treating with broad spectrum antibiotics can help rats survive secondary bacterial infections that can accompany a viral infection.
By studying how viruses are transmitted (via blood, air, and fomites) we can learn ways to help prevent the spread of viral infection. Proper quarantine is the best defense against harmful viruses.