The core of Mac OS X is BSD Unix, and that OS has been around for two decades in open source form, inspected by all concerned. That's why Mac OS X is more secure than Windows. For a long time, apologists for Windows have been arguing security through obscurity. However, if it has a CPU, hackers will try to attack it, and Mac OS X has been a big target for a long time.

Microsoft Could Learn OS Foundation Lessons from Apple