ClamAV is by default installed in Ubuntu. You want to be sure that you have the latest version and update.

Clamav will sometimes have trouble downloading updates for some reason or another. I’ll show you how to fix this in Ubuntu Linux! First, stop clamav and freshclam:

$ sudo /etc/init.d/clamav-daemon stop
$ sudo /etc/init.d/clamav-freshclam stop

Next we are going to manually download the latest signatures from clamav’s website and place them into the proper location in Ubuntu. In Ubuntu Linux, clamav signatures are located in /var/lib/clamav/.

$ wget http://database.clamav.net/main.cvd && sudo cp main.cvd /var/lib/clamav/
$ wget http://database.clamav.net/daily.cvd && sudo cp daily.cvd /var/lib/clamav/
$ wget http://database.clamav.net/bytecode.cvd && sudo cp bytecode.cvd /var/lib/clamav/

Next, fix some permissions:

$ sudo chown -R clamav:clamav /var/lib/clamav/

Finally, start clamav:

$ sudo /etc/init.d/clamav-daemon start
$ sudo /etc/init.d/clamav-freshclam start

Lastly you may want to check the clamav logs to make sure clamav is updating OK.

$ cat /var/log/clamav/freshclam.log

Bron: https://scottlinux.com/2012/09/04/clamav-signatures-not-updating-ubuntu-linux/

Many people prefer graphical user interfaces. That’s why we install a GUI.
Go to the following website, and look for the latest version of the *.deb file:
https://bitbucket.org/dave_theunsub/clamtk/downloads

Open the file in Software Centre. Click install.