A virus is a program that replicates. It spreads by making copies of itself on a computer or by inserting code in the files of applications or operating systems. Viruses are not always harmful for files or computers, but usually affect the performance and stability of the system.
To infect a computer or spreads, it is usually necessary to perform an action, such as opening an infected e-mail attachment.

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Reference: Shinynews Perhaps we talk about it more than we use it and still it isn't clear for all...