Possible Duplicates:
Can virtual machines infect their host?
Virtual Machine and Virus
Is the host OS isolated in terms of security? If I don't secure my VM could my Host OS be at risk? If it is secure then running windows in a VM on Linux may be the safest way to run Windows!
In theory it is possible if there is a vulnerability in the virtual machine. In practice, there's no need to worry as long as you keep the networking locked down -- don't let the guest have access to your internal network except the specific route needed to get to the internet.
Yes, it has been demonstrated. See also this answer. Although I'd agree that it is not very common yet.
