The program will run as usual, and everything looks normal for the user. This is a combination of different types of virus. They may infect boot areas and executable files, and may infect through a network. There is a separate paper about macro virus. This does not need a host, thus it is not a virus even though all other criteria are present.
Mainframes are the computer types most attacked by worms.
A worm transfers to other computers through the network. This causes system overload by constantly propagating through the network. A Trojan horse does not propagate. Therefore it is not a virus even though all other criteria are present. A Trojan's success is dependent on the users being na ve.