Probably only within the virtual machine itself. I can't see how a virtual machine running Windows can usefully infect the outside Mac OS with something.
Mmmm, I'd be careful with what sections of the Mac OS disk that you allow the Windows virtual machine to see.
It is quite conceivable that someone comes up with a Windows virus that scans to see if "Mac System"-looking directory structures are accessible, and then deposits a Mac executalbe malicious payload in a place sensitive to the Mac OS.