Computer viruses are programs that are written either to do harm to data or files, or to play a prank on unsuspecting computer users.
For detailed information about what computer viruses are please refer to: An introduction to computer viruses document.
Some viruses may delete documents and files from individual computers, network drives or web servers while others can try to use your computer to attack other computers or web sites. Some viruses, called hoax viruses, are harmless and may only display a silly message.
A virus can be spread through e-mail, usually via attachments, instant messages messages, or through the Internet via a security hole in an operating system The faster a virus can spread, the more dangerous it is.
You can't get a virus just by reading a plain-text e-mail message. What you have to watch out for are messages that include an executable file attachment (a file usually ending with .exe or .com) or that contain embedded executable code (like Java script in an html message). As soon as you open an infected attachment, you contract the virus.
Your computer is also vulnerable to infected Word or Excel attachments because these files can have a macro virus. Any shareware program that you download from the Internet ( for example, a game or screen saver) can also be infected. The risk for these types of viruses is much smaller, however if you have any doubts, save the document to a CD, DVD or flash drive and then have your antivirus program scan the disk before opening. When in doubt, delete the message or file, especially if it is sent by someone you don't know.