No viruses.
A Mac App Store with guaranteed clean applications.
No worries whatsoever.
And then...your favorite web browser suddenly seems to have a mind of its own; taking you places you have no interest in going and warning you of evils on your Mac that don't actually exist.
Over the last several months I've had several people report that their computers have been hijacked. This hijacking takes a variety of forms, but most often it's an inescapable barrage of ads or warnings of impending doom. In many cases these result in pop-up windows loading that can't be closed or navigated past. The screen shot in the upper right and the one below were taken from a client's computer in such a state:
This kind of browser hijacking attempts to create fear about an existing or impending problem on your computer and then offers a solution that consists of calling a toll-free number to get that problem resolved. At worst this is a phishing attempt or ransomware and at best it's an attempt to sell you software of dubious value that is supposed to "remove" the software causing the problem. In every case it's a pain in the arse.
(For an in-depth look at how these scams work, check out Lenny Zeltser's excellent Conversation With a Tech Support Scammer, which includes audio of conversations he had with "tech support" when calling one of these toll free numbers.)
Avoiding adware and malware is pretty simple:
Avoid other equally sketchy sites, such as torrent hosting services and... oh... you know you know what I'm talking about...
If you find that your Mac has been hijacked by Adware, not to worry, we've got a fix for you. The Safe Mac's Adware Medic. (The Safe Mac also has an excellent website and Twitter feed if you want the latest, up-to-date info on Mac Adware, Malware, and security concerns.)
Using the app is as simple as it gets.
Adware Medic can usually remove adware without requiring a restart of your Mac, but in some cases a restart will be required to fully remove any adware that was installed.
If Adware Medic doesn't resolve everything that ails your Mac, you can take additional steps to resolve these issues. In many cases these fixes may be as simple as avoiding certain websites, changing your broswer's home page and search settings, or looking at removing browser extensions you may have installed.