It hits Windows almost exclusively, because it's such a buggy poorly-designed POS.
isn't it true that NO unix based OS (which every OS is) is secure by definition
Every OS is not a *nix - see Windows or MacOS <= 9. That said, a multiuser OS like linux can be quite secure - install the software you need, then create a regular user (who cannot alter the system), disallowing execution of anything in that user's home dir. Pretty much no chance of anything happening then.
isn't it true that the least (so far) affected Operating Systems are also the least unpopular, because few hackers/spammers will bother to find/exploit weaknesses
I'm guessing you meant to say something else there.