I had almost a same experience when i were taught linux in my semester,i simply ask my instructor that "can a linux contain a virus ?".he says
No,Linux is virus free(yes that's why i like to learn by myself,he is doing the same job like a windows(infected)).what he says means linux is incapable of running programs and i quit attending classes(it will not effect my attendance as i am doing my course through a distance learning).as far as i know unix has various variants and probably they have different security models so it's difficult for a hacker(lot of work) to design a virus for different variants and along with these things unlike windows,password is must with a username in unix(so again they need to do some extra work).
And Campbell,
I have my reasons(everyone has) for using windows.but now i am trying to learn linux(so i just ask a question to @Steven,probably i have that right on ranch).anyway don't get me wrong with all these things.i have not any problem with your jokes,so you can continue again..
Kind Regards,
Praveen.