The Windows guys say when OS X becomes common, it will be affected by viruses just as much (the epidemiology standpoint). The Unix guys say, no, because Unix is designed as a multi-user system with restricted user and file permissions from the ground up, so viruses have far fewer avenues for propagation (the system design standpoint).
And then there are userland macro/script viruses, where the unix file permissions don't apply, a problem common to all pluggable applications, especially where the user is a) not asked to consent to execution of a b) digitally signed macro.
That's as much as there is to this debate. Simple, hmm?