Introduction <ul><li>A virus is a software and it belongs to the category of malware. It is able, once executed, to infect files in order to reproduce making copies of itself, usually without being observed by the user. </li></ul><ul><li>Those who create these viruses are virus writers. </li></ul>
What is a virus? <ul><li>A virus is composed of a set of instructions, like any computer program. It is usually composed of a very small number of instructions (from a few bytes to a few kilobytes), and it is specialized to perform only a few simple steps and is optimized for use as few resources, so as to be invisible as possible. The main feature of a virus is to reproduce and then spread throughout the computer each time you open the infected file. </li></ul>
<ul><li>a virus in itself is not an executable program </li></ul><ul><li>There are two phases of a virus: </li></ul><ul><ul><li>when it is only present in a mass medium (hard disk, floppy, CD, ...) the virus is inert, even if copied to your PC can not do anything until you run the hosted program </li></ul></ul><ul><ul><li>when it is loaded into memory the virus becomes active and begins to act. </li></ul></ul>
The virus better known: The Trojan <ul><li>Trojan or "Trojan horse" are generally malicious softwares (malware) hidden inside seemingly useful programs, and therefore the user runs a voluntary basis. The type of malicious software that will run silently in the enforcement of the file from the user may be a virus that has any kind of threat as science allows the hacher who has infected your PC to trace the ip victim. </li></ul>
<ul><li>This is an example of trojan virus captured during scanning with avg </li></ul>
Exchange of virus <ul><li>Nowaday many virus programmers, but especially in the past, have exchanged the sources of the virus to understand the new programming techniques. Many exchanges of virus occurred through websites called VX. VX means Virus eXchange. Today sites (at least those public) dedicated to the VX are few but we think that there are underground sites that contain the latest virus database accessible to crew virus writers only. </li></ul>
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