They spread to every disk you put in the computer. Most of the time, this means that the only way they can get into your computer is if you boot up from an infected floppy diskette. Parasitic viruses attach themselves to other programs. Whenever you run an infected program, it searches for other programs to infect. When people share utilities and games, they can easily pass viruses along with them.
Parasitic viruses can spread using diskettes, email, and downloads from the Internet.
Don't run programs from unknown sources, always run a virus scanner on new programs you get, and don't run a program attached to an email unless you're absolutely certain it's clean. Macro viruses , like the infamous Melissa, are the latest fad. Since a virus can only travel in a program, not in a data file, people always considered data files safe. The Melissa virus shows how sophisticated the reproduction of macro viruses can be. If it finds Outlook, then it sends email to the first 50 addresses in each address book you have.