Virus

"Virus" is also a: user

(thing) by CentrX (11 mon) (print)   (I like it!) Fri Jun 09 2000 at 16:39:16
Any of a kingdom (Virus) of prokaryotes, usually ultramicroscopic, that consist of nucleic acid, either RNA or DNA, within a case of protein. They infect animals, plants, and bacteria and reproduce only within living cells. Viruses are considered as being either living organisms or inert chemicals.
(idea) by Fluffy The Cat (2 y) (print)   (I like it!) Fri Jul 21 2000 at 23:55:57
Viruses can be surprisingly elegant things. While some, such as CMV, contain as much genetic material as some bacteria, others have the bare minimum to be able to enter a cell and reproduce. It's tempting to think of viruses as incredibly cunning organisms - in fact, it's probably more accurate to think of them as a self-sustaining chemical reaction which happens to involve animal cells. Viruses don't think, yet are capable of defeating a highly evolved immune system and everything modern science can throw at them. I find them both terrifying and strangely attractive. See prions for the logical continuation of simplifying infective bodies, and become somewhat concerned.
(thing) by jafuser (1.2 y) (print)   (I like it!) 1 C! Mon Oct 09 2000 at 4:07:39
ASCII Art drawing of a typical virus (in this case a T4 phage virus): 
    head            tail
 ____|____  _________|_________
'         ''               __  '
  ______                __/  __
 /      \              /  __/
/ (((((  \_|---------_|\_/
\  ))))) /"|---------"|/ \__
 \______/              \__  \__
          /   /      /    \__
  / collar   /      /            \
 /          /       \            tail fibers
/         sheath    base plate
\
 protein coat
 
 (((
  ))) nucleic acids (inside protein coat)

Typical cycle for the spread of viral infection:

  1. The virus attaches by it's tail fibers to a cell wall by recognizing certian features on the cell's receptor sites.
  2. The virus works like a syringe by breaking down the cell wall at the base plate, and then using the sheath to pump the DNA through the cell wall into the cell.
  3. The empty virus capsule (protein coat, sheath, etc) is discarded.
  4. The cell begins making hundreds of copies of the entire virus using the DNA it was injected with.
  5. The cell walls break down and release these viruses into the host.
(idea) by Jargon (2 y) (print)   (I like it!) Thu Jul 19 2001 at 17:49:19
virtual shredder = V = visionary

virus n.

[from the obvious analogy with biological viruses, via SF] A cracker program that searches out other programs and `infects' them by embedding a copy of itself in them, so that they become Trojan horses. When these programs are executed, the embedded virus is executed too, thus propagating the `infection'. This normally happens invisibly to the user. Unlike a worm, a virus cannot infect other computers without assistance. It is propagated by vectors such as humans trading programs with their friends (see SEX). The virus may do nothing but propagate itself and then allow the program to run normally. Usually, however, after propagating silently for a while, it starts doing things like writing cute messages on the terminal or playing strange tricks with the display (some viruses include nice display hacks). Many nasty viruses, written by particularly perversely minded crackers, do irreversible damage, like nuking all the user's files.

In the 1990s, viruses became a serious problem, especially among Windows users; the lack of security on these machines enables viruses to spread easily, even infecting the operating system (Unix machines, by contrast, are immune to such attacks). The production of special anti-virus software has become an industry, and a number of exaggerated media reports have caused outbreaks of near hysteria among users; many lusers tend to blame everything that doesn't work as they had expected on virus attacks. Accordingly, this sense of `virus' has passed not only into techspeak but into also popular usage (where it is often incorrectly used to denote a worm or even a Trojan horse). See phage; compare back door; see also Unix conspiracy.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

(thing) by WWWWolf (1.6 y) (print)   (I like it!) Tue Mar 12 2002 at 23:28:49

A computer virus is basically a snippet of code that replicates and ocassionally does something else. It typically uses some other computer program or executable area as the "host".

There are many different types of computer viruses.

Boot block virus
Virus that spreads from boot block to another. Somewhat common in the era when people still used floppies to boot computers commonly. (First PC/MS-DOS virus was called Brain, and it was a boot virus and the first stealth virus!
Executable virus
A virus that infects executables (binaries or scripts). Typically it attaches itself to the end of the executable and adds a jump instruction to the front.
Document viruses/Macro viruses
Spread via macros stored in documents, using an application as the host.

Most infecting viruses check if the host file already has an infection. (Sometimes not. In this day and age, gigabyteful executables might be shrugged at.) These virus "signatures" are often used as basis of virus detection (but in case of polymorphic viruses, new kinds of heuristics have risen. Sorry, my literature is from end-1990s. =( )

The age of huge computer networks has brought us...

E-mail viruses
These are bordering on a worm and virus (some experts call this a "hybrid threat") - some of these have traits of both. Typically, they spread as E-mail attachments; the users are lured to open infected document (see document virus) or run an executable (this is behavior closer to a normal worm, but since it needs manual steps to spread instead of being fully automatic, it's also viral in nature).

In fact, E-mail viruses are older than Outlook Express... (see Christmas Tree Virus.)

Legality of viruses: In most countries, it seems, writing and intentional spearding of viruses is considered dishonorable (if not by courts, at least by users!), in some countries it is downright illegal. (Making and intentional spreading is illegal in Finland - of all countries! And this happened in the country that has F-Secure...)

Oh yeah, and the eternal nitpick: Not long ago, the plural of 'virus' was whatever you damn pleased wherever you wanted, but these days, you need to malloc() it.

(thing) by Szlater (6.9 d) (print)   (I like it!) 2 C!s Sat Jun 29 2002 at 19:33:42

A virus is a non-cellular genetic element that hijacks a cell for its own replication. In its extracellular state the virus particle ,also known as a virion, is metabolically inert and does not carry out respiration or biosynthetic functions. After the infection of a cell, the virus genome is produced and the components that make up the virus particle's protein coat are synthesized from and by the cell's own structural and metabolic components.

The word virus originally meant any poisonous emanation, such as the poison from a snake. Louis Pasteur often referred to pathogenic bacteria as viruses. By the end of the 19th century, a large number of bacteria had been isolated and shown to cause disease, but there were some infectious diseases for which no bacterial cause could be shown. One of these diseases was foot-and-mouth disease, a serious skin disease in animals. In 1898 Freidrich Loeffler and Paul Frosch found the cause. They showed that the agent that caused foot-and-mouth could pass through filters that would stop any bacteria. They showed it was not a toxin by demonstrating that it could cause disease at very low dilutions and was transmitted through filtered material between animals. Over the next few years a number of filterable agents were shown to be the cause of various plant and animal diseases. They came to be known as filterable viruses, but as more work was done on them, the word "filterable" was dropped.

In 1915, the British scientist F.W. Twort discovered a class of virus that infects bacteria; the French scientist F. d'Herelle named them bacteriophages in 1917. Although bacteriophages are still viruses the name has stuck.

The virus genome can be either DNA or RNA based, in either single or double stranded form.

Virus Families

a. DNA Viruses

Double-stranded DNA

Bacteriophages:

Plant Virus Groups:
Animal Virus Groups:
Single-stranded DNA

Bacteriophages:
Plant Virus Groups:
Animal Virus Groups:
b. RNA Viruses

Double-stranded RNA

Bacteriophages
Plant Virus Groups
Animal Virus Groups:
Single-stranded RNA

Bacteriophages
Plant Virus Groups

Animal Virus Groups:

It is now well established that viruses can cause cancer. Certain viruses can bring about a genetic change that results in the initiation of tumour formation. This initiation event could be the activation of a proto-oncogene into an oncogene or the deactivation of a tumour-supressor gene. Once initiation has occurred the cell may remain dormant for some time, until an environmental change brings about promotion. Once a cell has been promoted to the cancerous condition, continued cell-division can result in tumour formation.

Although it is difficult to establish, it is now accepted that the following cancers have viral causes.

Dictionary of Biological Terms, 11th Edition. Lawrence, E (ed). Longman Scientific & Technical, 1995
Biology of Microorganisms, 7th Edition. Brock et al. Prentice Hall International, 1994
(thing) by Lord Brawl (19 hr) (print)   (I like it!) 3 C!s Fri Nov 08 2002 at 3:40:23

Overview

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.

Taxonomy

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.

Diseases

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

Footnotes

  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
(review) by avalyn (31.7 min) (print)   (