I own an iMac and I also use Windows so the security of both operating systems concerns me. There does seem to be an increasing amount of malware for the Mac. I don't know what that means. There is still little malware for a Mac but it has been demonstrated that malware can be written for Mac OS X. The worse was the Flashback Trojan that infected something like one out of every 100 Mac computers running Mac OS X.
My best guess based on experience is that Macs are somewhat safer than Windows but malware can be developed if the malware writers really want to develop that malware. So they must not be interested because there are so few Mac users compared to Windows users. But Mac users tend to be somewhat more wealthy than Windows users so I think the malware writers are missing an opportunity. Because my experience is that most Mac users do not use antivirus software and some do not even know how to turn on their firewalls.
Personally the first time I remember having any contact with malware was with Apple Computers before Mac OS X.
One thing that concerns me very much is that the hackers seem to be turning their attention to server computers. If servers can be attacked successfully that is a bigger achievement than just hacking a desktop personal computer. And we are seeing that servers can be hacked.
From what I have been able to find out vulnerabilities on Linux server computers that bring about Heartbleed and Shellshock are incredibly important and probably should be front page news. The hackers don't need to hack personal computers if they can get passwords, credit card information, etc., from the server computers.
And contrary to what some have said these vulnerabilities may not be patched. Heartbleed was still affecting many Linux servers months after it was first reported. So the servers were not being updated.
Shellshock is much more serious. Somebody somewhere probably needs to WAKE UP!