''A third situation occurs when the virus replaces some of its target, integrating itself into the original code of the target. Clearly, the virus writer has to know the exact structure of the original program to know where to insert which pieces of the virus. Finally, the virus can replace the entire target, either mimicking the effect of the target or ignoring the expected effect of the target and performing only the virus effect. In this case, the user is most likely to perceive the loss of the original program.''

please explain the above method of virus attaching itself to a program