virus
Infectious particle consisting of a core of nucleic acid (DNA or RNA) enclosed in a protein shell. They are extremely small and cause disease. They differ from all other forms of life in that they are not cells – they are acellular. They are able to function and reproduce only if they can invade a living cell to use the cell's system to replicate themselves. In the process they may disrupt or alter the host cell's own DNA. They use the cell they invade to make more virus particles that are then released. This usually kills the cell. The healthy human body reacts by producing an antiviral protein, interferon, which prevents the infection spreading to adjacent cells. There are around 5,000 species of virus known to science (1998), although there may be as many as half a million actually in existence.
Examples of diseases in humans caused by viruses are the common cold, chickenpox, influenza, AIDS, herpes, mumps, measles, and rubella. Recent evidence implicates viruses in the development of some forms of cancer (see oncogenes). Antibiotics do not work against viruses. The best protection against diseases caused by viruses is immunization.
Viruses can change by mutation. When they do so, a human body is sometimes unable to fight the new virus very well. This happens regularly with the influenza virus. A small change results in a small influenza epidemic, but a big change results in a pandemic that can kill millions of people worldwide. Many viruses mutate continuously so that the host's body has little chance of developing permanent resistance; others transfer between species, with the new host similarly unable to develop resistance. The viruses that cause AIDS and Lassa fever are both thought to have ‘jumped’ to humans from other mammalian hosts.
Types of virus Bacteriophages are viruses that infect bacterial cells. Retroviruses are of special interest because they have an RNA genome and can produce DNA from this RNA by a process called reverse transcription. Viroids, discovered in 1971, are even smaller than viruses; they consist of a single strand of nucleic acid with no protein coat. They may cause stunting in plants and some rare diseases in animals, including humans. |
| It is debatable whether viruses and viroids are truly living organisms, since they are incapable of an independent existence. Outside the cell of another organism they remain completely inert. The origin of viruses is also unclear, but it is believed that they are degenerate forms of life, derived from cellular organisms, or pieces of nucleic acid that have broken away from the genome of some higher organism and taken up a parasitic existence. |
| In 2003, research on the DNA of human faeces revealed the presence of over 1,200 different viruses that could be found in the human bowel, half of which were new to science. Most of the detected viruses belonged to the phage class, which specialize in killing bacteria. |
Antiviral drugs Antiviral drugs are difficult to develop because viruses replicate by using the genetic machinery of host cells, so that drugs tend to affect the host cell as well as the virus. Acyclovir (used against the herpes group of diseases) is one of the few drugs so far developed that is successfully selective in its action. It is converted to its active form by an enzyme that is specific to the virus, and it then specifically inhibits viral replication. Some viruses have shown developing resistance to the few antiviral drugs available. |
Occurrence of viruses in water Viruses have recently been found to be very abundant in seas and lakes, with between 5 and 10 million per millilitre of water at most sites tested, but up to 250 million per millilitre in one polluted lake. These viruses infect bacteria and, possibly, single-celled algae. They may play a crucial role in controlling the survival of bacteria and algae in the plankton. |
virus
In computing, a piece of software that can replicate and transfer itself from one computer to another, without the user being aware of it. Some viruses are relatively harmless, but many can damage or destroy data. Antivirus software can be used to detect and destroy well-known viruses, but new viruses continually appear and these may bypass existing antivirus programs. The earliest was the Brain virus, written in Pakistan in 1986.
Viruses are written by anonymous programmers, often maliciously, and are spread on the Internet, on floppy disks, CD-ROMs, and via networks and e-mail attachments. Viruses may be programmed to operate on a particular date. Most viruses hide in the boot sectors of floppy or hard disks or infect program files.
| Since 1995 there have also been macro viruses that infect Microsoft Word or Excel. The first of these were the Concept virus (infecting Word files) and the Laroux virus (infecting Excel). |
| In 1999, the Melissa virus caused an estimated US$80 million worth of damage, with LoveBug causing US$7 billion worth and affecting about 45 million computers the following year. In 2003, Slammer caused cash machines to crash and delayed airline flights, while SoBig and Blaster exploited weaknesses in the Windows 2000 and NT operating systems. Most recent viruses are actually worms, which replicate themselves over the Internet. |
| A recent ploy is to use ‘social engineering’ – such as the pretence of the message having come from a reputable company, or to be reporting a failed attempt to send an e-mail – to trick the recipient into opening the attachment containing the malicious code. |
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