BOAC:
As others have so correctly stated: I wouldn't worry about av on linux.
It is a rarity when any Linux distro suffers from virus infection. Don't give it another thought.
As for a firewall. Following this link and behold the joys of configuring iptables, if you so desire, or other methods of installing a firewall on your ubuntu distro. (Ubuntu is one of the few Linux distros which do not come with a default firewall installation.)
Basic Ubuntu Linux Firewall Configuration - Techotopia