Macro viruses

Macro viruses currently account for about 80 percent of all viruses, according to the International Computer Security Association (ICSA), and are the fastest growing viruses in computer history. Unlike other virus types, macro viruses aren’t specific to an operating system and spread with ease via email attachments, floppy disks, Web downloads, file transfers, and cooperative applications.

Macro viruses are, however, application-specific. They infect macro utilities that accompany such applications as Microsoft Word and Excel, which means a Word macro virus cannot infect an Excel document and vice versa. Instead, macro viruses travel between data files in the application and can eventually infect hundreds of files if undeterred.

Macro viruses are written in "every man's programming language" -- Visual Basic -- and are relatively easy to create. They can infect at different points during a file's use, for example, when it is opened, saved, closed, or deleted.