Viruses are one of the five types of microbes1. Unlike bacteria (q.v.), viruses are non-cellular.

A single virus particle, called a virion, consists of genetic material inside a protein shell (see Structure, below). In this stage a virus is, in essence, inert chemical matter. A virus infects a host cell, injecting its genetic material into the host cell, which is then converted into a virus factory. The cell makes many copies of the virus, which then break down the cell wall and are released to repeat the process with other cells.

Viruses have been classified both as living and nonliving at different times. Scientists still have no definitive answer. Only their hairdresser knows for sure!

Some common viral forms are listed under Diseases, below.


Not being classified as life in the normal sense, viruses are not classified in the normal system of Kingdoms. Virus taxonomy does not use the Latinized binomial system of Carolus Linnaeus used in biological taxonomy.

The International Committee on Taxonomy of Viruses (ICTV) has proposed a standard system of viral classification. The "Seventh Report of the International Committee on Taxonomy of Viruses" recognized more than 1,550 virus species belonging to 3 orders, 56 families, 9 subfamilies and 233 genera.

Classification is based on factors such as:

  • The type of nucleic acid, RNA or DNA;
  • The shape of the capsid;
  • Whether the nucleic acid is single or double strand;
  • Whether the capsid is enveloped by other cell material, or not;
  • The presence of sense or anti-sense nucleic acid;2
  • The type of host cells infected.

A bacteriophage, sometimes simply called a phage, is a virus that infects bacteria.

Structure and form

A virus consists of nucleic acid3, 4 within a protein shell, which is called a capsid. Viruses have one of three basic forms: polyhedral (such as the polio virus), helical (such as the Tobacco mosaic virus), or complex.

Bacteriophages can take complex forms, often with a protein appendage attached to the capsid. This appendage is used to inject the virus genome into the host bacteria.

The T4 bacteriophage is one such complex form. (See jafuser's excellent ASCII diagram, above.) It looks like a microscopic robot spider or the Apollo project's lunar lander, with an icosahedral capsid containing the viral genome, mounted on a rodlike appendage, to which are attached several other, leg-like appendages. The legs are used to secure the T4 to a bacterium, and then the rod injects the viral genome into the cell.

Most viruses are extremely small, measuring approximately 15 to 25 nanometers in diameter, with an observed upper limit of 150 nanometers for large spherical forms. Hepatitis C, as an example, is approximately 50 nanometers in size.

Reproduction (Replication)

Viral infection can spread from injection into a cell to complete replication of multiple new viral particles in a 20 to 30 minute time span. Particularly virulent forms, such as Dengue fever, produce numerous copies of the virus per cell and can rapidly infect any host body.

A typical virus replication cycle5 (the lytic cycle):

  • Adsorption - The virus attaches to a cell wall. Certain proteins in the virus react with cellular receptor sites. (Cells which lack matching receptors are not susceptible to the virus.)
  • Penetration - The virus breaks down the cell wall, and then injects its genome like a syringe through the cell wall. The now-empty capsid is discarded.
  • Uncoating - The viral genome discards any protective coating inside the host cell.
  • Replication - The cell's own DNA is destroyed by the invading genome. The cell begins copying the virus genome, and new virions are automatically assembled. (This copying usually occurs mainly inside the cell nucleus in eukaryotes.)
  • Assembly - New virions assemble from the replicated parts.
  • Release - The cell wall breaks down and the virions are released into the host to repeat the cycle. Often the host cell is killed, but in some cases it may not be, allowing the virus to remain dormant and recur again later.

In some cases, called "the lysogenic cycle" (or "pathway"), the viral genome does not begin copying itself immediately. Instead, it inserts itself into the DNA of the cell, modifying the chromosome. It is then copied along with the cell's normal DNA in numerous generations as part of the bacteria's normal reproductive cycle (via binary fission). This lysogenic state is stable, but not permanent. Later, in response to a change in conditions, induction occurs which (re)activates the lytic cycle.

The discovery of how a virus incorporates itself into a cell's DNA and be copied along with it laid the groundwork for genetic engineering, in which viral techniques are used to modify the genome.


Viruses can cause many diseases in humans. Here are some of the more common viral diseases:


  1. The others are bacteria, protozoans, fungi, and helminths (worms).
  2. The anti-sense strand is the second strand in double strand DNA or RNA, which can be used to 'switch off' genetic actions.
  3. A single or double of either RNA or DNA, but not both types together. RNA viruses are more commonly plant viruses, while DNA viruses are commonly animal viruses.
  4. This genetic material is a key difference between viruses and prions, as prions contain only proteins.
  5. Please forgive the overlap with jafuser's excellent writeup, which didn't quite say what I needed.

Primary references

  • Other writeups in this node. This writeup began life as high school biology lecture outline. Some redundancies exist with other writeups in this node, both for completeness and because they were used as a source for this work. Care has been taken to minimize the redundancies.
  • http://biology.about.com/library/weekly/aa110900a.htm
  • http://www.kcom.edu/faculty/chamberlain/Website/Lects/PROPERT.HTM
  • http://www-micro.msb.le.ac.uk/335/VirusGroups.html
  • http://www.sparknotes.com/biology/microorganisms/viruses/section1.html
  • http://homepage.smc.edu/ishida_carolyn/Week12/Week14ViralRep.htm