I have a Windows 7 partition on my MBP that I installed with Boot Camp. I have reason to believe that there was a virus on my Windows 7 partition (did some scans, got some sketchy results from Avira). I decided to just wipe the entire partition using Boot Camp Restore to reformat the old partition and add it back to my OS X partition. I'm wondering however if in the time period I had the two partitions up a virus could have jumped from the Windows 7 partition onto the OS X partition, in which case I now need to worry about a virus on my OS X installation?
The virus needs to be able to access the HFS+ partition on which OS X is installed. If you do not have software that permits access to such partitions on Windows (such as Paragon HFS+ For Windows [no affiliation]), it should not be possible for the virus to end up on the OS X partition.
Well, let's say, a virus/worm spreading from one platform/OS to another is quite unlikely.
However the comments here, saying it would be impossible, are simply wrong. It is perfectly possible to equip malware with the ability to operate cross-platform. Neither different binary formats nor file systems are insurmountable barriers. It's just adding extra effort in the development phase.
And that is pretty much the point, it is probably not actually worth the effort for a malware author to implement that.