Many Linux users, and non-Linux users, believe Linux is a virus-free OS.
Other believe that even if a virus hits a Linux machine, the impact would be little, as it runs without root privileges.
So the shocking news are: both wrong. Linux distros has bugs and vulnerabilities which can be exploited for malicious activities. Such can include remote-control trojans, rootkits, data-theft, and so on. Those viruses gains root access without the user's acknowledgment.
Recently I found at work a server hosting tons of Linux viruses, with the source code, which exploits recent kernels used in modern distros. Here are few examples.
I don't want to raise any panic. Linux is still way safer than Windows. So does OS X. This is mostly due to the fact those OSes are far less popular on the desktop. Also, updates are released faster, and thanks to package management, installed regardless which piece of software is vulnerable.