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Computer virus life-cycle. The above image is 400 pixels across, the original measures 2,600 pixels across.Computer virus life-cycle. Suggested uses: computer virus outbreaks, hackers, the biology of computer viruses, computer worms, how computer viruses spread.
Computer viruses are auto-replicating code inside machines. They are usually designed to cause damage to data. Other related entities may extract data from machines for spying or theft. They can slow machine performance and corrupt data.
Computer viruses are viruses in a very real sense of the word. The overriding characteristics of any virus are that it replicates (and not much else) and requires another system to replicate in. Such a "host" system furnishes the virus with the raw materials that it needs to replicate. This means that all viruses are completely dependent on their host in order to replicate and so are known as "obligate parasites". In the case of biological viruses that replicate inside living cells (e.g. animal cells and plant cells) they are known as "obligate intracellular parasites". These viruses use cellular machinery, energy and raw materials to build copies of themselves. In the case of computer viruses, the host computer (or more generally any machine) provides the energy (electricity) and the machinery to perfom the replication.
Growth of viruses: viruses don't grow or develop in the way that a nomal organism with a body does but they can still evolve. Viruses are essentially constructed in their final state. What we normally think of as a biological virus (the icosahedral shell) is really just protective coat or capsule that contains the viral information (genome) as it moves from cell to cell.
General definition of a virus: viruses are self-replicating information packages. The information can be stored in any medium (DNA, RNA, binary computer code). The replication requires a host system (cell or machine). Following replication the viruses spread to new hosts.
Are so-called biological viruses really living things?: Viruses have many of the attributes of life. They replicate (with a greater or lesser degree of copying fidelity). They can evolve by natural selection. They are intimately associated with living systems. They are made from standard biological building blocks. Some people regard them as alive whereas others regard them as non-living. They are classified by a standard biological taxonomy.