Daniel: It depends on the particular virtual box and whether is is configured to allow the virtual environment to have access to the host system. If it has no access then the host is secure. If it has read-only access, then data on the host could be vulnerable. If the virtual system has write access to the host, all bets are off.
They can hack your router, change your DNS, and redirect the host system to a malicious site.
They can install a virus into your memory to infect whole system.
Kaspersky Labs awhile back got a new virus themselves. They rebuilt the OS, still it was there. They swapped out the entire hardrive and still infected.
Finally they found a high tech malware infected their memory, something they never saw before, and they use virtual machines to test new viruses.
As long as you're using a natural connection you should be fine, but it you want to be real secure run vm in sandboxie.
Depends which base OS you are using.