yes you can infect the host o/s,
there are dns changers that can interact with systems on your network, so you would need to isolate the vm from your network
the guest will still be using your router to connect to the internet, the same router your host is using
http://blog.trendmicro.com/dns-changer-malware-evolves-again/
shared files between the host and guest can infect either - there are virus's that are vm aware, they can detect that they are running in a virtual environment so become dormant, the user is tricked into thinking its clean so they transfer files across - bang, instant infection of the guest,
some will hunt out the shared folder, this gives access to the host
http://forums.virtualbox.org/viewtopic.php?f=2&t=24687
http://www.technibble.com/forums/showthread.php?t=14350
http://superuser.com/questions/29611/can-a-virus-from-a-virtualbox-vm-affect-the-host-computer
i would advise that you read those links, most are for virtualbox but you will get the idea