Computer viruses usually spread in one of three ways: from removable media; from downloads off the Internet; and from e-mail attachments.
Although the Internet gets a bad rap as a source of viruses, you're no more likely to contract a virus from the Web than you are from packaged software. Still, scan everything you download, and update your antivirus software regularly.
E-mail is not the virus breeding ground it's made out to be, either. In fact, it's nearly impossible for a virus to be transmitted by plain-text e-mail. Most viruses can only spread via attachments — either rich-text e-mail or attached applications. Using antivirus software, scan attachments from people you know, and never open attachments from people you don't. If you're a Microsoft Outlook user, you can also select security preferences that keep e-mail-borne viruses from exploiting the close relationship between Outlook and the Windows operating system.
For information on specific types of computer viruses, check out Understanding Computer Viruses.
These precautions will minimize the risk of infecting your computer as well as keep you from spreading viruses onto others. If you find that your computer is infected, make sure to read Responding to and Recovering from a Virus for tips on recovery.
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