Anti virus is not needed on Ubuntu, any of its flavors, or any Linux distro. There are no known Linux viruses in the wild. What exists is proof of concept viruses that require you to do something stupid like run as root to work.
Anti virus eats up resources and gives a false sence of security.  The only people running Linux that need a anti virus application are those that run mail servers that allow Windows boxes to connect to them. This is so that the Windows boxes cant send viruses to other Windows boxes. More of a favor to the Windows user as even mail servers are imune to the viruses.
For the most part those that install anti virus are those that are leaving windows and think its just a normal thing to have. It isnt, viruses are the result of the poor design and security that is Windows. Be glad this is something you can leave behind.