Viruses

A virus is a computer program designed to copy itself into other programs, with the intention of causing damage. A virus will usually execute when it is loaded into a computer's memory. Once a virus has been downloaded, it instructs its host program to copy the viral code into local programs and files. The danger is that the corrupted programs could continue to perform their intended functions while also executing the virus's instructions, further propagating it. A harmless virus may simply cause a cryptic message to appear when the computer is turned on; a more damaging virus can destroy data. Antivirus software can be used to detect and remove viruses from a computer, but the software should be updated frequently for protection against new viruses. Such programs, including Norton AntiVirus or Mcaffee can be easily downloaded from the internet. Alternatively, students can download VirusScan from the AUP Computer Lab.

Source
http://www.britannica.com/ebc/article?tocId=9361324&query=virus&ct=