Antivirus should not be needed for Ubuntu. Linux is very secure, and there are very few viruses out there written for it, with most viruses being written for Windows or Mac.
The fact that most basic programs for Ubuntu are available through the default repositories in the Ubuntu Software Center also adds to the security. Programs found here have been verified as safe and have been tested on every version of Ubuntu they can be installed on.
Make sure you stay up-to-date with the Software Center, avoid downloading unknown
.debs or tarballs (
tar.gz usually), stay away from untrusted PPAs, and you should be perfectly fine.
Further reading: http://www.howtogeek.com/135392/htg-explains-why-you-dont-need-an-antivirus-on-linux-and-when-you-do/