Anecdotally, I have never had a virus on Mac (that I detected) and neither did any friends, but Windows got infected all the time. This was true going back to Windows 95.
My point is that both are very vulnerable, precautions are what keeps you safe.
Anyone remember when you could click Cancel in the login screen and get access to the desktop? Good times. :)
Did they really have viruses (of which for OS X there are very few and far between, to the point of lore) or trojans?
If you're getting infected all the time using Windows, you're doing something stupid. The operating system provides the vulnerabilities, but you might want to reassess how you're using your software.
Or, you know, just switch to an OS where you don't.
http://www.macnn.com/articles/16/06/01/researchers.show.that...
http://www.zdnet.com/article/lenovo-rootkit-ensured-its-soft...
http://www.makeuseof.com/tag/now-three-pre-installed-malware...