Building a realistic model for a network defense system against a worm attack is vital to better understand the effects of a worm attack on network assets and functionality. Traditional epidemic worm modeling does not take into consideration the real network topology or network actual defense measures. In this paper, the authors reviewed the network defense systems from different perspectives for defining the level of immunity of different parts of the network and ascertaining the real impact of a worm attack on the network. The idea of immunity came from examining and comparing the immune system in the human body to the defense and security measures of computer networks. Then, they developed a novel, realistic model by splitting the network into the highly immune part of the network (HIN) and the partially immune part of the network (PIN) in order to measure the real impact of worm attack on computer network. Next, authors evaluated the effectiveness of this model by implementing network defense measurements adopted from the human immune system. Computer simulations show that the infection waves of worms in HIN have minimal impacts compared to those in the PIN.