Mac OS is not based upon Linux. Darwin is based upon Unix, while OS X's userland is very similar to BSD.MAC OS is based on Linux and MAC OS is a linux OS. Get it right.
They could re-active the web cam (if they know how to) but most likely, that won't happen.But if they infect your computer with a RAT, then they'll be able to re-enable the webcam, and view live footage via an attack using programs such as metasploit? (Correct me if i'm wrong)
Most 'hackers' or people that create these viruses are generally quite experienced within that department, and will have the knowledge and determination to get this and potentially commit Identity Fraud or start to blackmail you. Nice thread thoughThey could re-active the web cam (if they know how to) but most likely, that won't happen.