In theory a mean/vicious virus can corrupt a partition table; thus taking down the entire hard drive without regards to OS installed. There is nothing you can do about it, but if you just recreate the partition table you should be able to get all your data back.
Additionally, in theory, a virus could be clever and have it's own access to the mac partition and delete files there. But I say in theory because it would be a major undertaking by the virus writer. I have never heard of one doing this either. But in theory it is possible.
And finally, if the virus does sector writes directly to the hard drive, it can wipe the entire hard drive. 🙁 Again, although possible and it has been done, it isn't often to be done.
What we do know for sure, is that no virus can install itself into files on the Mac partition and thus infecting your mac. That hasn't happened, and I don't see it happening anytime soon.
