Whilst Microsoft Windows and Android are the most popular areas for Malware as they are by far the most widely used operating systems I dont think you will find anyone is immune:
You are right, nobody's immune, but you've got to agree that the chance of malware is higher with an open system. There are pluses and minuses to a closed system such as Apple's, this is one of the pluses.
However, it is possible to have the best of both worlds e.g. the Mac App Store gives reassurance in this area, but if you really want to go into the wild west you can install an unsigned app. I have to say it would be nice if Apple would give this same choice on IOS, I am grown up enough to make my own decision thank you