When you run the infected program file, the virus is copied into the operating system. It will then infect other program files which are run which allow the virus to be transmitted. To be successful, a virus must have an incubation period. If you are aware you are infected, you will not inadvertently transmit the virus to other computers. This is a nuisance virus which runs on the 18th of each month.
Viruses are platform dependent.
That is, a Windows virus will not infect a MacIntosh. Worms Worms are viruses specially designed to be spread along networks. Normal viruses spread by moving floppy disk from computer to computer. Macro Viruses Macro viruses are the most common and most annoying viruses at the moment. Most of them have been written for Microsoft Word.