Some viruses are relatively harmless, but others can damage or destroy data.
Viruses are written by anonymous programmers, often maliciously, and are spread on floppy discs , CD-ROMs and via networks. Most viruses hide in the boot sectors of floppy or hard discs or infect program files, but more recently there has been a huge growth in macro viruses that infect Microsoft Word or Excel. Antivirus software can be used to detect and destroy well-known viruses, but new viruses continually appear and these may bypass existing antivirus programs. The Computer Misuse Act of 1990 made the release of computer viruses an offence.