If I get a windows virus on my mac can it infect my VM? I know that a virus on my Vm won't affect my mac.
Highly unlikely. The virus that could harm your VM from the host side would have to be a Windows virus that's somehow functional under OS X AND be able to manipulate the VM's HDD. Never say never but for now this is so specialized that I wouldn't consider it being a risk. You still can easily raise an additional wall in this scenario: Just add encryption to your VM (configuration/safety) - that should prevent any virus from directly editing your VM.
