Viruses spread from one computer to another via the sharing and distribution of infected software or infected disks. Viruses may be malicious or non-malicious. Non-malicious viruses replicate, but they do not attempt to do anything destructive. Malicious viruses intentionally damage or delete files or destroy the contents of hard drives. Non-malicious viruses can be damaging, even if the damage is unintentional.
Viruses occupy memory and disk space which can cause unexplained crashes and strange behavior.
No virus is completely harmless. Among the viruses one of the most common is the Macro virus. They are written in the macro language of applications such as Microsoft Word, and can potentially infect any platform that can run the application. Many other applications have macro languages, so the problem is likely to be a growing one. Viruses can be spread by transferring infected documents and data files as e-mail attachments.