For reference, I am defining these things here so you can understand the difference:
Scareware: malicious computer programs designed to trick a user into buying and downloading unnecessary and potentially dangerous software, such as fake antivirus protection.
Malware: software that is intended to damage or disable computers and computer systems.
Adware: software that automatically displays or downloads advertising material (often unwanted) when a user is online.
Virus: a piece of code that is capable of copying itself and typically has a detrimental effect, such as corrupting the system or destroying data.
Ransomeware: a type of malicious software designed to block access to a computer system until a sum of money is paid
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Personally, I don't use any, nor do I scan for anything. There are currently no "viruses" running on the Mac platform, although there is an increasing amount of malware and adware floating about. Plus, lots of scareware (MacKeeper and the like).
The two most common issues we are seeing in the shop are adware and scareware. The most important thing to note is that none of it gets to your Mac all on its own (unlike the computer virus epidemics that plague Android and Windows). Adware and scareware turns up mostly while users are surfing adult websites. They get popups (adware) warning them that unless they download (or call) this "scanning/cleaning" software, their Mac will explode (scareware). These two forms are almost always in bed with each other, i.e. one leads directly to another.
I have seen some ransomeware on Macs, but only running on Windows within virtual machines. However, because VM's like Parallels and Fusion create disk-sharing so Windows can move files back and forth between itself and Mac OS, the ransomware is able to encrypt the Mac volume and leave the entire system unusable. Last week, Macrumors
reported on some ransomware being propogated by a BitTorrent client, so there are instances of this crap running on the Mac.
As far as Macs propagating viruses on a network, I have not personally seen it in action, although it certainly is technically possible. However, if I understand it correctly, a virus would first have to exist on the Mac that was capable of copying itself across your network. 0's and 1's don't spontaneously make copies of themselves; a piece of code must be running to copy them from point A to point B. There has been some Malware for the Mac (downloaded through infected,
pirated copies of Apple's Pages and Adobe Photoshop, IIRC) that were capable of this. It seems that this was about 7 or 8 years ago, though.
Many moons back we did sell Intego's Anti-Virus software for the Mac (taking advantage of people's completely unreasonable fears—shame on us
), but we discovered it was circumventing some built-in security features and causing trouble for our customers, so we took it off the shelf. We haven't carried anything since then, and warn our customers that installing any of the fool stuff is just asking for trouble.
I can tell you not to be fooled by
any of the ads or other nonsense you might encounter while surfing about. It is
all scareware/adware/malware and isn't worth your time.
However, I can very highly recommend that you install the
AdBlock Plus extension (if you are using Safari) which will clear up all the click-ad and pop-up-ad Adware you will likely encounter. Also, if you're concerned about malware (the only real threat Mac users face), you can install Malwarebytes from
Adwaremedic. This seems like a good tool, and we've been installing it in the service department for about a year. We have had no blow-back from customers, so we trust the product.
I think the more likely outcome in your situation would be for some random Word or Excel document to be harboring a virus. As the .doc or .xls file can reside on your Mac in its complete form, it would be possible for someone to fileshare into your Mac, grab said document and transfer it to their Windows machine, thus propagating the virus. I suppose something like Norton Anti-virus for Mac would discover said file and remove it, although the offending Word document may go along for the ride (although, perhaps, Norton is capable of removing the offending gibberish without destroying the file?). Then, when you discovered the file was missing, you might be seduced into downloading it, again, and start the process all over.
I take a hands-off approach to Macs and viruses: I don't worry about it and refuse to contaminate my system with the resource-hogging garbage known as Anti Virus. However, some network admins may require it, so you might have no choice. If you have a choice, don't burden your system with it. After all, the Windows machines on your network should be running it, right? Aren't they then inoculated and supposedly immune?
As for your Mac getting infected from a Windows machine, the chances are slim at best, and may be just short of "never". We have been doing data migrations from virus/malware-plagued Windows machines for years. Every user-generated file is moved—including emails, photos, documents, those silly .exe and .uix files that show up all over the user's documents folder, mail attachments and random downloads, you name it. Not once, and I mean
never, has a piece of Scary Software™ ever had a negative affect on the Mac in question. Never, ever, not once! I would worry more about someone hacking the network and rummaging around in my documents folder than I would about malware moving from Windows to Mac. Important: Malware, scareware, adware, viruses and ransomware running on Windows machines will be executables which the Mac won't even recognize as legitimate software and won't run, anyway. You have little (if anything) to fear.
When a customer brings a troubled Mac into the shop, one of the first things our techs do is look for instances of MacKeeper, anti-this-or-that, and any other computer "cleaner/scanner" software (including anti-virus). We inform the customer of the need to remove it and do so. Such things always result in improved performance (although said software may not be the sole cause of trouble, but often is).
The short version of this post goes like this: Install at your own risk!
