I don't fully understand how Macs can't get a virus. Does Microsoft care about this?
They run different operating systems and so the coding is COMPLETELY DIFFERENT.
A virus is, at its core, a program. A program designed for windows (PCs) will not work properly on a machine that runs the OS that Mac uses.
Thus, viruses coded for windows will ONLY work on Windows enabled computers; viruses coded for Macs will ONLY work on Macs.
Since the VAST majority of computers in the world run on Windows, people who create viruses pretty much only code for Windows, thus Macs don't have to worry *as much* because it's usually not worth the time to code a Mac virus.