A system or boot-record virus works by infecting the first sector of every DOS formatted disk. Generally a boot sector virus executes its own code, which infects the hard drive, and then continues the normal boot up process. There are also multipartite viruses that infect both boot sectors and files. In addition some viruses are labeled stealth , because they attempt to hide from detection, or polymorphic , because the replicants are different from the original, making them harder to detect. Lastly are the macro viruses.
Macro viruses are very special, and as a result are much more of a threat than normal viruses.
Unlike other viruses that are written for a specific operating system on a specific platform, and attach themselves to an executable program, macro viruses are written in WordBasic. This has several ramifications. First, WordBasic is readily accessible and macros are relatively easy to write. Second macro viruses infect documents, not executable files. Document sharing or moving is practiced much more commonly than the sharing of executable programs, therefore these may spread more rapidly.