It comes with a Firewall but you need to turn it on:
You may want to install 'clamav' or get a free copy of 'bitdefender' for linux - you get this by going to their site and registering your e-mail. Viruses are not for Linux but to protect from passing potential payloads that someone has emailed in an attachment.
You may also like to install 'chkrootkit' to monitor potential rootkit intrusion - rootkits are NOT platform specific and can affect any OS.
If you visit a site and download a compressed package (tar.gz for example) you need to check its contents first before extracting as it could run a script that could wipe your Linux install off your machine - as happened with a bogus Linux theme from gnome-look.org over a year ago called 'Ubundows'. I did copy instructions how to do this. courtesy of Blackwolf from ultimateeditionoz.com.
Also I would advise that you only have your external drive attached whilst off-line and doing backups off-line - there is a new threat around the corner based on crypto locker that can identify what make and model of external drive is attached and you then get faced with a ransom note if you want to access your data again! (Not sure if this is only Windows users that get affected!)