Zellon has written
I don't think much of it. Just don't execute anything strange. If my knowledge is correct... a virus can't just jump on your system anyway.
Yes, it can.
In theory you're partially right. On a perfect system, no virus could be run without the command of the user.
But all programs contain bugs and some (for example buffer overflows) allow the unallowed execution of program code, for example if a faulty image library loads a malicious image in your browser.