No computer system is absolutely secure!

Example: Stuxnet -This virus was, supposedly, created to infect the systems in Iranian nuclear plants. It infects windows systems and lays dormant until it finds a specific micro controller, in this case the one that controlled Iranian centrifuges, and hijack them. Critical systems such as nuclear centrifuges are not connected directly to the internet, yet because this virus, known as a worm, burrowed itself into so many PC's, it eventually found itself on its target via and infected USB drive.

Gas Centrifuge