There are many 'experts' who think that if you are very carefully you will not ever be infected by any kind of bad virus, worm or Trojan or something like that.
The truth is that all AV software is based on the premise that we already know what the virus will look like or what it can do. That is a false assumption. Nobody has calculated how many different ways there are to compromise a PC or any other kind of electronic information transport system.
But this is not said here to discourage you from being careful. No, always be careful. The more effort you take to protect your PC puts the odds in your favor.