Condensed from Linux.com
First and foremost, there is no such thing as a secure system. As Joe ‘Zonker’ Brockmeier says, security is not a feature, it’s a process. Having said that, it follows that Linux is not immune to everything. Unlike Windows, you don’t need an antivirus with Linux but should you want one, you can install ClamAV. As always, prevention is better than cure so be proactive. Install Ksplice, it’s free for Ubuntu and Ubuntu-based distros. Understand user permissions and encyption. For server-based computers, use chkrootkit, Tripwire or Snort.