Unix, Linux, System and Network Security.
Do a little Cyber-victimology. What OS do Botnet nodes run? Where do viruses run?
...and it came to pass that all Things were replaced by Computers-in-Cans-pretending-to-be-Things, and these Computers-in-Cans ran Software, and it was Bad Software. The End.
Asking Smart Questions is a good read.
Q&A for Ubuntu users and developers
Q&A for users of Linux, FreeBSD and other Un*x-like operating systems
Q&A for professional and enthusiast programmers
Q&A for users and developers of hardware and software for Raspberry Pi