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Viruses are one of a number of (ordinarily)malicious \1oftware, so named (obviously) because they appear to mimic the spread of disease in living things. Malicious, in this context can mean many things, including doing damage to a system, stealing information, and even simply leveraging the system to do damage to other systems. Viral software can function and spread on any system where the operating system and other security software isn't sufficiently protecting the system.
Another name for this type of software is malware. Akin to viruses are things like trojans, keyloggers, and bots.
On the other hand, these things don't always act maliciously. Keylogging and bots can be useful security tools when deployed in defence of the system by its owner(s).