Anti virus software protection

I read Anti Virus Protection FREE softwares on collegelib, http://www.collegelib.com/t-anti-virus-protection.html and installed it. I am curious, how does it work? why anti virus itself is not getting virus affected? Can anyone explain me this?

One of the reason could be: Constant Updates.

The Anti-Virus companies keep on updating their products so that it stays updated and give less chance to be affected.

My Avast gets updated everytime i get connected to internet. So that it makes me stay safe as well as the software.