Some viruses are harmless but many are troublesome. Historically it has been known or suspected that individuals called Hackers are responsible for originating computer viruses. These Hackers are usually smart individuals who want to access other computers either with the purpose of curiosity or with the intent of acquiring or using information. Computer viruses work in your computer or on you disk by setting themselves up so when you try to access a program you are sent to the virus. You cannot obtain you information because the virus has changed the accessing information on your disk or computer.
Some viruses work by bombing a server or site with unwanted mail.
This creates the server or site to be overloaded. Hackers can access sites by doing this and they can obtain information. Some viruses can be sent via email on the net. For example this is how the Melissa virus works. When you open an infected document, if you use Microsoft Outlook as your email system, the Melissa virus will send a copy of itself as an email attachment to the first fifty people in your address book.