A virus is a program or code that is capable or penetrating systems, disks or individual files usually without the owners knowledge or consent. Once a virus has successfully infected the target further action depends on the specific type of virus, but may include:

• Only replicating within the infected system
• Infecting further files as they are accessed or created
• Erasing or corrupting data in host systems/files
• Waste resources without causing outright damage

Today, the term virus is used both to refer to classic viruses and as a generic term for malicious programs including classic viruses, worms, Trojans and so forth.

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• About viruses