The look like any other macro in the file, but when the file is opened, the virus executes commands understood by the macro language. Macro viruses can exist inside any document whose application uses a macro language, such as the Melissa of a few years ago. Melissa was passed on through Microsoft Word documents. All a user with a clean version of Microsoft Word had to do was open an infected Word document, which then infected the application. (How Data Gets Infected 1-2) .
Finally, the multipartite viruses have characteristics like the boot sector viruses and the file viruses. They can start out in the boot sector and then spread to the system's applications or the other way around. (Types of Viruses 2) There are other programs called "worms" or "Trojan horses" that are not viruses but have the same effect. These worms replicate themselves by e-mail, "making use of any Outlook address books" (Types of Viruses 2).
Aside from introducing viruses through the system's applications, hard disc or floppy disc, there are other ways for the system to become infected. There are incidents of viruses finding their way into a system through licensed, shrink-wrapped software is added to the system. Generally, but not always, this software is safe, but occasionally the virus has been introduced in the original handling of the disc. (How Your Data Gets Infected 1) But the main ways viruses enter the a computer system are "through files added to the system by way of removable media such as floppy discs or Zip disks and from downloading from the Internet. It is possible to get viruses from e-mail attachments and, rarely, through a plain text e-mail message alone. The Kak worm is one of those rare exceptions. It hides itself in the signature file of an Outlook Express e-mail message. Just opening an infected e-mail is enough to infect the system.