We often hear that viruses do not affect Linux systems. If it was only true... To understand why there is malware in the first place, we look at the reasons for evildoers to create harmful software. When that is clear, we move on by defining several types of malware, to finally focus on a very particular one, the rootkit. A quick course into the cleverness of rootkits follows, with the related challenges it offers for detection. We close the session by giving tips on detection and prevention.