Viruses are the bane of a Windows user's existence. Between Outlook viruses, Outlook Express viruses, data-deleting viruses, Trojan horses, and any number of other nasty things transmitted from one computer to another, it's almost impossible (and certainly unwise) to use a Windows computer without an antivirus utility. Fortunately, the Mac OS (both OS 9 and OS X) is fairly virus-free. Windows viruses generally take advantages of technologies or security flaws in Windows to do their damage, so the Mac OS isn't susceptible. And although there are Mac viruses, they're very rare. This means that whereas antivirus software is a requirement on a Windows PC, on the Mac side the issue is more one of "Am I at risk?"
If you don't download a lot of new, strange software, don't really exchange data (CDs, Zip disks, floppy disks, etc.) with other Mac users, and don't receive lots of e-mail from Windows users, you're actually at a pretty low risk for viral infection. Most viruses are transmitted via e-mail attachments (which is why I told you at the beginning of the chapter never to open an attachment you didn't ask for), infected downloads, or files swapped with infected computers, so if you don't interact with any of these things, chances are you're pretty safe.
However, if you do tend to come in contact with these types of files, a good antivirus utility like Norton AntiVirus (http://www.symantec.com/nav/nav_mac/index.html) or Intego VirusBarrier (http://www.intego.com/virusbarrier/home.html) will ensure that you don't catch the latest computer flu. These utilities automatically scan downloaded files and removable media, monitor e-mail attachments that you receive before you open them, and even watch for suspicious activity on your hard drive. If they find a virus, they "disinfect" it, or else warn you not to open the infected file. Both also check for the latest virus definitions over the Internet to make sure your antivirus software can catch even the newest bugs.
Mac users regularly receive Windows viruses as e-mail attachments. Although these viruses can't hurt your Mac, they can still hurt the Windows PCs used by your friends, family, or business associates! Good antivirus software like the two mentioned here also prevents you from passing on a Windows virus to your Windows friends.
In addition to traditional viruses, there are also viruses that infect Microsoft Office files. These files, called macro viruses—which are actually much more common for Mac users than traditional viruses—take advantage of the macro programming language available in Word and Excel to infect your computer. Word and Excel actually provide you with a warning when you open a document that contains macros—the one you probably always ignore (I know I do). Thankfully, most antivirus utilities also protect you against macro viruses.