provided by: 
Last week, in my Norton AntiVirus Begone! entry, I offered instructions for removing Norton AntiVirus from a Mac. That entry appeared in this week's Macworld Weekly Newsletter and prompted a slew of messages along these lines:
When I purchased my Mac I was told getting a virus on a Mac is unlikely and that I did not need protection. What do you recommend?
Calm and care.
By calm I mean that the Windows world is so rife with viruses, adware, spyware, and other varieties of malware, it's a commonly held belief that all computers are susceptible to these kinds of cooties and one would be a fool to operate a computer without some kind of prophylactic utility. This belief is manifest in Windows users who move to the Mac and immediately purchase antivirus software in the belief that it's a necessary part of owning a computer.
And, on the Mac, it's just not.
The hedge for those of us in the recommendation business is that while there may not be viable virus threats now, there could be one day. In order for us to cover our respective patoots, we must follow up our "Nah, you don't really need antivirus software" suggestion with "-at the moment, but someday you might" and then we weasel and waffle in anticipation of the day when The Bad Thing Appears and the villagers appear at our gates with blazing torches and the more rustic form of pitchfork.
And by care I mean that it's worth your while to learn the difference between the various cooties that some people too-broadly define as viruses. As in:
Virus This is a program that earns its name by its ability to replicate itself, locally and often across a network. Many viruses attach themselves to other programs. When those programs are launched, the virus code is launched as well and the virus goes about its nefarious business.
Viruses are commonly found in the Windows world. Not in the Mac world. Viruses are where we most often employ the "not now, but maybe someday" antivirus weasel.
Trojan A Trojan (shortened from Trojan Horse) is a kind of malware that promises one thing but delivers another. For example, you've downloaded an application that promises to make you rich, cure male-pattern baldness, and double the size of your ring finger. When you run this miracle worker the contents of your computer are, instead, beamed to an underground data center in Kamchatka.
The greatest risk in this scenario is the person operating the computer. If you obtain software from reliable sites such as VersionTracker and MacUpdate and resist the urge to open a file you receive in an anonymous email message, you're unlikely to get one of these Trojans.
If you'd like an extra measure of security should something rude find its way onto your Mac, consider purchasing a copy of Objective Development's $30 Little Snitch. This handy utility will alert you when an application attempts to send information out from your Mac. You're welcome to approve applications that are doing the right thing and apply the hairy eyeball to applications that appear to be up to no good.
And if you're also concerned about incoming traffic, enable your Mac's firewall (found in Leopard's Security system preference).
Adware and spyware Adware is software that has an embedded advertising component-one that displays or downloads ads when you run the software. Some adware is legitimate-part of the price of using a "free" application such as Twitterific or Eudora, for example. Spyware is malware that grabs data from your computer and often uses it for the purposes of evil-sending personal information to a baddie or, when using your web browser, redirecting you to sites you don't want to visit.
In order for the worst forms of this kind of malware to work, the operating system must allow unrestricted access to its more sensitive parts. The Mac OS doesn't and so adware and spyware are not currently something for Mac users to worry about.
Phishing Like Trojans, phishing schemes-those schemes that trick you into revealing personal and financial data-exploit the weakness of the person sitting at the computer rather than the computer itself. These are fraudulent offers or warnings that arrive via email or instant message demanding that you provide credit card, social security, password, or bank account information in order to maintain an account or service or confirm a transaction.
For example, you receive a message from your credit card company suggesting that you confirm your username and password in order to continue using the bank's online services. Click the link that supposedly takes you to the bank's website and you're presented with a webpage that looks exactly like the real deal. But, of course, it isn't. Provide the information they seek and woe is you in the form of a drained bank account or massive credit card bill.
To thwart phishing schemes all you need do is avoid taking the bait. Banks, lending institutions, credit card companies, auction sites such as eBay, online services, Internet service providers... any reputable outfit that holds personal information never demands this kind of information in the form of an email message.
Ah, but what about Windows on a Mac?
A lot of people are running Windows software on their Macs-either with the help of Apple's Boot Camp or through a virtualization product such as VMware Fusion or Parallels Desktop for Mac. Regardless of how you run Windows, the operating system you're running is the real deal and is just as susceptible to computer cooties as a PC running Windows. In this case you would be a fool to skirt antivirus software in the Windows environment.
So, do you need antivirus and/or security software? I've chosen to do without it and I've yet to regret that decision. But then I try to practice safe computing. If you're the kind of person who clicks questionable links and opens suspect files without hesitation, leaves your network unprotected, and uses "password" as your Administrator's password, perhaps you could use a little extra protection in your life.
Read article at Macworld.com