The macros have automated functions that are useful for easy infection. This allows the virus to spread into the computer as soon as the Word or Excel data file is opened. Viruses that have that ability are network and macro viruses. These are examples of multipartite viruses. Some other common network viruses are Homer, ShareFun, Xerox PARC, and Wank Worm.
Most viruses have two phases to their existence, the infection phase and the attack phase.
All viruses have an infection phase, but not all have an attack phase. During the infection phase, the virus spreads itself. If a virus infects too fast, it is usually easy for anti-virus programs to spot. Therefore many try to be subtle about it. Viruses can be spread by innocent people that are just doing their daily routine.