Just like in the Mac world most of the malicious stuff require user to mistakingly execute them on their own.

Avoid bad applications. Don't install anything outside ubuntu's own repositories unless you know what it is or it has been verified trusted and is used and recommended by trusted parties (often including ubuntu or canonical)

this is common sense but it still needs to be repeated to people because the current trend of thinking is that simply having linux makes everything go away. You still need your common sense. Oh and get ClamTk and GUFW for your ubuntu. (antivirus and simplistic firewall GUI).
Get firestarter for more in-depth firewall GUI.

This gets a b*mp even if the post doesn't exist. These rules have always applied and will always apply. Just change a few names to make them fit your OS/distro.