A computer virus is a program that has the ability to replicate, or make copies of itself, and spread to other files. Viruses can attach themselves to many types of files and programs, and spread as files that are copied and sent from one computer to another. Just like biological viruses, computer viruses spread quickly and can be difficult to eradicate.
Viruses are never accidents, or caused by computer glitches.
All viruses are created deliberately. While some virus payloads merely display messages or images, others can destroy files, reformat your hard drive, or send unintended email messages. Even if a virus doesn't contain a damage routine, it can still cause trouble by wasting storage space and memory. Additionally, some viruses are poorly written, and may cause a computer to stop or damage files. Many viruses make the computer's memory unstable or cause programs to run improperly.