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.
Learned to program in 1967 (Algol-60), multiple architectures, multiple languages and assemblers, got into security while learning Perl (complained about Joe accounts in corporate NIS, was remembered when company had breakin), stayed because security is fun. Never ran Windows! CDC G-21, IBM 1401, Univac 1108w/Exec II and VIII, DECSystem-10, -20, VAXen, Unix boxes, Amiga, Linux. Reasoning techniques: WTF?,RTFM, Zoom/Mooz, enblobification, WhatIfIBelievedThat-WhatWouldIHaveToDisbelieve?, boxes and tubes (tubes connect boxes that act/fail, open box to see more boxes and tubes), open the box that fails for examination. Keep going until you find the box that is good on only the input side. Fix that one.