For those of you who don't need or want an "active" solution, try
Bitdefender Antivirus Scanner from the App store; it`s free, nonintrusive, and runs only when you want it to. The scanner does not offer much in the line of protection being very much an on demand tool, equally for the majority of OS X users, most just want to validate that the drive is free of malicious code be it related to OS X or Windows. Where Bitdefender`s Scanner excels is detection & simplicity having no daemons or start up agents etc.
You get Bitdefender`s top notch detection in a basic package that you control, however this is not for those who are looking for a set & forget solution, as all scans are manual, and updates only occur when you open the application. That being said an Apple Script, Automator can easily trigger the application at a given interval. The scanner learns and only scans new & modified files, so in general running a scan for malicious code is swift & painless, once the first pass is competed. Custom scans and Drag & Drop area all present.
Depending on usage/workflow an "active" Malware application is not always the best solution, and there is also the argument that such "active" solutions can be a double-edged sword potentially increasing the "attack surface" against sophisticated threats. System security is very much a multifaceted beast, malware prevention/detection is for both OS X & Windows just one aspect, which is always worth thinking on
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