Definition of Viruses
A virus is a type of program that can replicate itself by making
(possibly modified) copies of itself. The main criterium for
classifying a piece of executable code as a virus is that it spreads
itself by means of 'hosts'. A virus can only spread from one computer
to another when its host is taken to the uninfected computer, for
instance by a user sending it over a network or carrying it on a
removable disk. Additionally, viruses can spread to other computers
by infecting files on a network file system or a file system that is
accessed by another computer. Viruses are sometimes confused
with worms. A worm, however, can spread itself to other computers
without needing to be transferred as part of a host. Many personal
computers are now connected to the Internet and to local-area
networks, facilitating their spread. Today's viruses may also take
advantage of network services such as the World Wide Web, e-mail,
and file sharing systems to spread, blurring the line between viruses
and worms.
Viruses can infect different types of hosts. The most common targets
are executable files that contain application software or parts of the
operating system. Viruses have also infected the executable boot
sectors of floppy disks, script files of application programs, and
documents that can contain macro scripts. Additionally, viruses
can infect files in other ways than simply inserting a copy of their
code into the code of the host program. For example, a virus can
overwrite its host with the virus code, or it can use a trick to ensure
that the virus program is executed when the user wants to execute
the (unmodified) host program. Viruses have existed for many different
operating systems, including MS-DOS, AmigaOS, and Mac OS; today,
the majority of viruses run on Microsoft Windows.
A legitimate application program that can copy itself as a side-effect
of its normal function (e.g. backup software) is not considered a virus.
Some programs that were apparently intended as viruses cannot
reliably self-replicate, because the infection routine contain bugs.
For example, a buggy virus can insert copies of itself into host
programs, but these copies never get executed and are thus unable
to spread the virus. Self-replicating programs that have very limited
spreading capabilities because of bugs are sometimes not considered
as being viruses.
Viruses in human body and computer
Microscopic, simple infectious agent that can multiply only in living
cells of animals, plants, or bacteria. Viruses are much smaller than
bacteria and consist of a single- or double-stranded nucleic acid
(DNA or RNA) surrounded by a protein shell called a capsid; some
viruses also have an outer envelope composed of lipids and proteins.
They vary in shape. The two main classes are RNA viruses
(see retrovirus) and DNA viruses. Outside of a living cell, a virus is an
inactive particle, but within an appropriate host cell it becomes active,
capable of taking over the cell's metabolic machinery for the production
of new virus particles (virions). Some animal viruses produce latent
infections, in which the virus persists in a quiet state, becoming
periodically active in acute episodes, as in the case of the herpes
simplex virus. An animal can respond to a viral infection in various
ways, including fever, secretion of interferon, and attack by the
immune system. Many human diseases, including influenza, the
common cold, and AIDS, as well as many economically important
plant and animal diseases, are caused by viruses. Successful
vaccines have been developed to combat such viral diseases as
measles, mumps, poliomyelitis, smallpox, and rubella. Drug therapy
is generally not useful in controlling established viral infections, since
drugs that inhibit viral development also inhibit the functions of the
host cell. See also adenovirus; arbovirus; bacteriophage;
picornavirus; plant virus; poxvirus.
