Then you are in for a big surprise. Most of the viruses make their way into the system through where?
*drumroll*
Internet Explorer!
It has nothing to do with the fact that Windows is the most used OS in the world. It's a bad design that leaves Windows vulnerable to virus attacks.
It was Microsoft's biggest mistake to integrate Internet Explorer with Windows. That's why it's so easy to get infected with a virus while you use Internet Explorer. Safari is a standalone application that has nothing to do with the internal system of OS X. Internet Explorer is a part of Windows' internal system. It's like a back door that can be easily opened by anybody.
When it comes to Microsoft, quality control isn't the first that comes to my mind when I think of them.
Here's a good article on a virus author saying how difficult it is to develop a virus to attack OS X. It's actually a malware, it only replicates itself. The problem is, it's restricted to the directory it is in. So it can't do any damages to the system.
Mac virus author admits coding difficulties: News - Security - ZDNet Australia
From what I know, there are approximately 25 million OS X users. Maybe more. So out of all these users, there's still not a single harmful virus. Care to explain why?
It has nothing to do with the population of users. OS X is apparently well-designed if it has managed to remain virus-free since it was first released around 8 years ago or so.