Viruses written in the internal macro language provided with many applications. Macro viruses are extremely common—especially within Microsoft® Excel® and Microsoft Word files. They spread easily because they travel in documents that are often shared and also because many of them, in wormlike fashion, e-mail themselves to everyone they find in an address book. Because macro viruses are so easy to write and modify, new ones pop up all the time. Additionally, because macro viruses spread within an application, they may spread between operating systems—for instance from a PC to a Mac®.
During the late 1990s and early 2000, macro viruses were the most prevalent viruses. Unlike other virus types, macro viruses are not specific to an operating system and spread with ease via email attachments, floppy disks, Web downloads, file transfers, and cooperative applications. Popular applications that support macros (such as Microsoft Word and Microsoft Excel) are the most common platforms for this type of virus. These viruses are written in Visual Basic and are relatively easy to create. Macro viruses infect at different points during a file's use, for example, when it is opened, saved, closed, or deleted.