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.

Function

The primary function of a virus is to infect host cells and create more viruses. The virus does this by taking over the host cell's protein and genetic material-making processes, forcing it to produce the new viruses. Exactly how viruses function in this manner is best understood by examining general viral structure, classification, and reproductive strategies.

Structure and classification

There are three basic structures for standard viral capsids: icosahedral, helical, and complex. Icosahedral capsids are 20-sided, made of triangular capsomere subunits. The points of the triangular subunits join at 12 vertices about the shape. Although exact structure varies from virus type to virus type, a common arrangement is five or six neighboring triangular subunits at each vertex. Some viruses show more than one capsomere arrangement within the capsid. An example of a virus having an icosahedral structure is adenovirus, the virus that can cause acute respiratory disease or viral pneumonia in humans.

The helical viruses have protein subunits that curve about a central axis running the length of the virus. The fanlike arrangement of protein forms a three-dimensional ribbon-shaped structure that covers the viral genome. Some of these capsid structures are stiff and rodlike, while other helical viruses are more flexible. The influenza virus is an example of a virus with a helical capsid structure.

The third type of virus capsid structure is called complex. Although the structure is regular from virus to virus of the same type, the symmetry is not patterned enough to be fully understood. For example, poxvirus, the virus that causes smallpox in humans, has a complex capsid structure of over 100 proteins. Virologists are still trying to determine the exact arrangement of these proteins.

The combination of the capsid and the viral genome is known as a nucleocapsid. Some nucleocapsids are infective in this form and are known as naked viruses. Others require a surrounding lipid membrane derived from the host cell to be infective. The membrane envelope can encompass one or more nucleocapsids and usually contains on its surface at least one viral protein in addition to the host cell components. Viruses of this type are called enveloped or coated viruses.

Viruses are classified according to structural characteristics such as whether the virus genome is made of DNA or RNA. Both of these nucleic acids can form ladder-like structures where each side of the ladder is known as a strand. Viruses are differentiated by whether the DNA or RNA is single or doubled-stranded. The type of capsid structure and whether the virus is naked or enveloped are also considered. A few viral classifications take into account differences in replication strategy.