ClamAV is an open source (GPL) antivirus engine designed for detecting Trojans, viruses, malware and other malicious threats [http://www.clamav.net/].

To install, in Terminal type
<code>
sudo apt-get install clamav clamtk
</code>

It's also good to install a firewall. The 'Uncomplicated Firewall’, otherwise known as UFW, is designed to be very simple and is packaged with an intuitive user interface. The developers created it because they 'wanted to create a server-level firewall utility that was a little bit more for //human beings// [http://gufw.org/].

To install, in Terminal, type
<code>
sudo apt-get install gufw
</code>