A simple way to explain it - If an infected file is copied from a PC to a mac, the malware/spyware lays "dormant". When the file is copied from the mac to another PC, then it becomes active again. Not though that this does not apply to macro viruses in MS Office, as those can still cause issues on macs.
You can get them attached to files and pass them on to people with PCs but they will not affect your computer. One thing that I wonder about if you install crossover on your mac to run windows programs can you get viruses that will affect your mac?
I think it's important to note that there's no spyware yet. There is a mistaken belief that OS X is bulletproof. It isn't. It's tougher than previous versions of Windows (and likely Vista), there are fewer people trying to hack it, and we've been lucky.
So far, no spyware. But that does not mean there never will be.