Computer viruses come in many varieties but they are all designed to create digital mischief of some type. Viruses may be designed to steal passwords and account information, delete or rename critical system files, gain complete control over a personal computer or virtually anything else that the programmer desires. Viruses are often installed inadvertently by computer users who believe that they are downloading useful software or that the software they are downloading is virus free without double checking. Luckily the bulk of viruses are inert due to programming errors but there are also plenty of harmful viruses.
Computer viruses have been around since the 1970's.