The Linux malware is increasing (it's a good sign, because this means that Linux is becoming very popular!) and is a good practice to have a firewall enabled and a good antivirus to check the suspect files; this will also help to protect the other computers in your home/office.
To update the ClamAV's engine+database and fix vulnerabilities/bugs execute this safe and easy procedure:
Connect to the internet; you need a stable connection.
In a terminal (xterm is ok) execute
sudo apt-get update and
sudo apt-get upgrade. This will ensure that your system is up to date; the clamAV installer can behave differently if your base installation is old or if your source definitions are old.
Install the latest ClamAV from the Official Ubuntu repository, like any other Ubuntu software. Do not install it from other, unsafe and non-specific sources, it can be dangerous.
sudo add-apt-repository ppa:ubuntu-clamav/ppa and then
sudo apt-get upgrade. This should update the engine.
After all that, execute
sudo freshclam to update the definitions. You must do that after an engine upgrade.
If you can't upgrade automatically your ClamAV installation, upgrade it from the source code or wait for an Ubuntu-specific release. I often suggest to compile the packages from sources because you often obtain a better result and you see what you are installing; please note that this type of installation is not always possible, you need the correct dependencies.
To install ClamAV from sources follow the Official ClamAV Source-Install Guide; if you don't understand one point of the guide, feel free to ask by commenting under here and don't forget to press the UP arrow if I'm of any help.