There is a newer (10/2010) answer to this same question: Mac Q & A: Do I Need Antivirus For My Mac? and if you run OS X 10.6 Snow Leopard, Apple has included antimalware sofware in your system that runs in the background. You can read about it in this MacWorld article.
It’s kind of complicated, but much of the answer has to do with how you use your Mac.
First of all, I don’t run antivirus, but I have experience in what might be bad and what isn’t. But you can use your Mac in the same way, because antivirus is only one part of your security system. The other aspects are common sense to not download and install something bad. Solely depending on antivirus to keep you safe isn’t a good strategy; it pays to be aware of the threats out there and how they work their way onto your Mac.
This is what Apple advises in their most recent statement, which is all over the web on blogs: “The Mac is designed with built-in technologies that provide protection against malicious software and security threats right out of the box. However, since no system can be 100 percent immune from every threat, running antivirus software may offer additional protection.”
And that’s true: OS X is pretty safe from infection and hacking, but it’s not perfect, and things may very well change in the future. Apple has a pretty good record of getting security fixes out, but they have been criticized for being slow to address vulnerabilities that malware can take advantage of. large parts of OS X is based on open source software and as a result has many people looking at it and fixing issues.
If you cruise around the blogs and Apple forums you’ll read that many people say they’ve never used antivirus on their Mac; or that they do, but only to keep from passing on Windows bad stuff to Windows-using friends; or that they always have run antivirus on OS X but have never seen a Mac virus. Or they would never again touch antivirus for their Mac because they once used it and it messed up their Mac. In other words, your mileage may vary.
What kind of Mac user are you? Do you delete spam email and never open emailed attachments if you don’t know who they are from? Do you ignore the flashing spyware warnings on Web sites and never click on them? Do you keep track of the software you install and are aware that typing in your administrative password means you are installing something? Then you might not need antivirus with the good common sense you have.
Tech writers and security experts have been predicting the end of OS X’s near-malware free state for years. It all depends on market share, they say. Windows has 80-90% of the market, and that’s where the malware is, because that’s where the money is for the criminal element behind malware. And that’s where all the kids are who write viruses and trojans, because they can use point and click software for making malware for Windows.
But because Apple’s market share is increasing, OS X will become a target of malware. There will be money in it, or bragging rights for the virus writers who get the first one out, or what have you.
A virus travels and replicates inside attached files and doesn’t need to be installed to cause damage. Trojan Horses are programs that must be installed to run and cause damage, and you will know you’re installing a program when OS X asks for your administrative password.
There hasn’t been a Mac virus in the wild for years – and none for OS X – but there are some Trojans out there. And that’s the key: be sure you know what you are installing. Viruses and Trojans are as much a social and knowledge issue as much a technical one. Know what to do and what not to do to be safe and you’re most of the way to being protected.
In the last few weeks some OS X malware has been discovered, bundled into pirated copies of Mac software. Once again, that’s not a virus but a program that was put together with the installer package. But you’re only going to get that if you downloaded pirated software or someone gave you a CD with the installer on it.
Use common sense: don’t casually download and install software from the Internet, especially video codecs and other helper programs that Web sites say you need to watch a video. There have been instances of those tricks in the past, and I’m sure they will occur in the future. Those icons of those trojans will look like real OS X programs, with installers that are the same as real programs.
And don’t be intimidated or convinced by those fake error messages and virus warnings and spyware infections those popup menus and boxes tell you. They’re all for Windows, anyway. There could be one for OS X in the future, but that’s one more reason to use your common sense.
I’m not going to prognosticate about what might happen with OS X. I think more Mac malware will show up in the future, but I think common sense will still be a form of protection. I think Apple will continue to keep up with security updates that stop malware infections, and possibly Apple will start producing their own antivirus software.
So that’ a long answer to the questions about whether you should have antivirus on your Mac. If you’re a simple home user, you probably don’t need it. It’s an expense and one more thing to keep track of. I’ve taught my individual clients to be careful and most of them don’t run antivirus.
Business clients are another matter; most of the time they do run antivirus due to other liability concerns, because antivirus will stop Windows malware from traveling to Windows users in the office and among their own clients.
I do know that if there is ever a spike in malware for OS X, it will be big news and we will all hear about it. That might not change the way I work, but it may change concepts of safety for others. And it might change your judgement, too, on needing antivirus for your Mac.
And if you want to run antivirus anyway (which will protect you and also help prevent passing on viruses to Windows users, too), you can try the free and reputable Sophos Antivirus for Mac. See https://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition.aspx
Two links: one from Walter Mossberg at All Things Digital: Running Antispyware Software on a Mac …The Macintosh isn’t inherently invulnerable to malicious software…. There have been practically no viruses, spyware or other malicious programs written for the Macintosh that have actually spread outside the laboratory….For that reason, most Mac users don’t run security software, and security software companies don’t make much of an effort to sell it for Macs. I don’t believe it is necessary, so far, for all except the most paranoid (and those who run Windows on their Macs). In fact, freedom from the burdens of running and updating security software has been one of the Mac’s big advantages.
And from MacWorld: Antiviral Armor – Mac antivirus software assessed: reviews of Mac antivirus software, if you are interested in getting antivirus.