Can I get Virus on Windows Host from Linux Guest?

[y2ken]

I am running Ubuntu Linux 'Guest' OS on my Vista Ultimate 'Host'. I am not sharing any folders between the two. Can a virus infect my Windows host system through the Linux guest? I had assumed not, but now I'm not so sure. ????
I've gotten those fake "you are infected" warnings while browsing with Ubuntu that won't let you navigate away or close the window unless you agree to install their bogus anti-virus program which I'm sure would REALLY infect my computer. I usually just shutdown my VM rather than risk a problem.
I know that Linux is practically impervious to viruses because most viruses are written specifically for Windows. But I'd rather be safe than sorry.

[vbox4me2]

Its not easy but possible yes, how and other issues is something to discuss on the appropriate AV forums, therefor locking it here.