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A Computer Virus is simply a program software with some peculiar characteristics: it is selfreplicant,
it is selfexecutable and it often performs damaging actions that are unpredictable from the user,
generally unaware of the virus presence. Often the computer virus used another program to merge with
the same one and to enter in an anonymous way into the operating system, that supervises all the basis
computer operations, with the purpose of infecting and modifying it. The program that allows this intrusion
is called "Host Program". Today the computer viruses are thousands and of various tipology, as for example
the Cluster Viruses, the Kernel Viruses, the Worms, the Trojan Horses, the Polimorphic Viruses, the Macro
Viruses, etc. A virus written for an operative system does not have any effectiveness on systems different from
that one. Generally the viruses do not have effectiveness on the Hardware, that is the physical part of a computer.
In order to fight the infection of computer viruses it must use the so-called Antiviruses, that are always some
software programs. The Antiviruses cancel or limit the damages caused from the viruses and often destroy them.
For every kind of virus it is necessary the specific antivirus, written purposely for this. Viruses and Antiviruses
are often identified from fantastic names and nicknames. Historically it seems that the first surely well-known
virus is that one called "Creeper", that it appeared in 1970 in the Arpanet network, making to appear a message
on the monitors of the terminals connected in the network. The antivirus used to kill it was called "Reaper".
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