Viruses Health Article

Definition

A virus is an infectious agent, often highly host-specific, consisting of genetic material surrounded by a protein coat.

Description

Viruses infect virtually every life form, including humans, animals, plants, fungi, and bacteria. So small that they cannot be seen by a light microscope, viruses range in size from about 30 nanometers (about 0.000001 in) to about 450 nanometers (about 0.000014 in) and are between 100 to 20 times smaller than bacteria. As of the seventh report of the International Committee on Taxonomy of Viruses (ICTV), published in September 2000, known viruses have been assigned to 1550 species in 53 different families. Hundreds of other viruses remain unclassified due to lack of information.

All standard viruses share a general structure of genetic material, or viral genome, and a protein coat, called a capsid. The viral genome is made of either deoxyribose nucleic acid (DNA), the genetic material found in plants and animals, or ribonucleic acid (RNA), a compound plant and animal cells use in protein synthesis. The protein capsid is made of repeating, often-identical subunits known as capsomeres. Viruses are not strictly free-living, as they cannot reproduce on their own. Instead, they use host cell machinery to make both the viral genome and capsids of the newly formed viruses, or virions.

The broad category of viruses also includes unusual infective agents that are missing one or more components of standard viruses. These unconventional viruses include viroids, which exist as circular RNA molecules that are not packaged, and prions, infective particles that contain protein and little or no nucleic acids.

Some viral infections can cause damage to the host cell, resulting in disease to the organism. Other viral infections appear to make the host cells divide uncontrollably, causing the development of cancer. However, many viral infections are asymptomatic and do not result in disease. There are no cures for viral infections, due in part to the difficulty of developing drugs that adversely affect only the virus and not the host. Accordingly, preventative measures such as vaccines play an important role in the treatment of viral diseases.