Every computer system ever made, regardless of OS, can be breached. That is the simple fact. Windows OSes are by far the most targeted because of its sheer prevalence in the OS space. Linux and Macs mostly fly under the radar of malware writers, currently, because as a percentage of the overall OS market they are a small niche. When/if that dynamic ever shifts with increased market share=greater numbers of users, malware writers now driven by profit(s) will respond accordingly.