Does Ubuntu need anti-virus? This is a question posed by many of the legions of new users who try out Ubuntu Linux everyday. New users dive into Linux booting up the massively popular Ubuntu Linux.
Now for a quick background check; Ubuntu is stable, easy to use and a rock solid desktop. I have been using it since the Warty Warthog (Ubuntu 4.10 ~ 2004), it runs on all my systems: home server, virtual servers and laptops.
Back to the question:
An Ubuntu Virus?
, the short answer is no there is no significant threats to an Ubuntu system from a virus. There are cases where you may want to run it on a desktop or server but for the majority of users, you do not need antivirus on Ubuntu.
Keep in mind that while you don't need anti-virus does not mean you don't need to be security aware.
Members of the Ubuntu community have put together an excellent introduction to Security on Ubuntu Linux. There are also free firewall test and other scanning tools available to ensure your network is correctly configured.
Once you have familiarized yourself with the concepts and information in the guide; if you are really keen (or paranoid) I would suggest a security addition to your systems, it is known as Host Based Intrusion Detection system. My HIDS agent of choice is ossec.net, it will not detect a virus as such but it does alert you to anomalous behavior on the system by examining system logs and watching the file system. If you chose to run OSSEC you probably do not need to run rkhunter and chkrootkit that are mentioned on the Basic Security Wiki page.
This old install guide I did for OSSEC on Ubuntu has the basic steps in getting it up and running.
Finally if you have a need for running anti-virus on Ubuntu, there is a good article on the Ubuntu wiki that has links to the popular and free antivirus software available (such as AVG, Avast, Avira) and the open source clamAV.