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.
Viruses will not go away anytime soon. More than 30,000 have been identified and hundreds of new ones are created every month. With numbers like these, no one who uses computers is immune from viruses. Viruses can infect any type of file that contains executable code. Today, many other file types can contain executable code such as word processing and spreadsheet documents that use macros, which are simple embedded programs.