A virus is a type of destructive code or program that, when executed, reproduces itself by customizing other computer programs and inserting its own code. When this replication succeeds, the affected areas are then said to be contaminated with an infection.
Viruses typically spread by creating a copy of themselves on a user's computer system without the user's understanding or consent. Oftentimes, infections will be spread out by email attachments or by files shared on peer-to-peer networks. As soon as an infection has contaminated a computer system, it can spread out to other computer systems on the very same network.
Some infections are configured to spread themselves in a manner that might trigger major damage to a computer system. For instance, an infection may erase files, fill a hard disk with ineffective information, or corrupt system files. In many cases, a virus may even display a message or image that is developed to terrify the user.
https://scarabnet.org/?p=1022 Most viruses are reasonably safe, and just cause minor annoyance to the user. However, some viruses can be extremely dangerous, and can cause extensive damage to computer systems.