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== Viruses ==

A virus is a piece of code that executes on a computer system without the knowledge of the computer's owner. Viruses replicate themselves, attach themselves to executable files and are often able to spread to other system on a network. A virus usually arrives on a computer attached to an executable program as a ''payload''. When the user executes the program, the virus starts up and begins to work. A common way for a virus to gain access to a system is via email attachments. Viruses can be categorized as follows: