They activate every time the infected file is executed by copying themselves into other executable files and can remain in memory long after the virus has activated. Thousands of different file infecting viruses exist, but similar to boot sector viruses, the vast majority operate in a DOS 16-bit environment. Multipartite Viruses Multipartite viruses have characteristics of both boot sector viruses and file infecting viruses. Macro Viruses Macro viruses currently account for about 80 percent of all viruses, according to the International Computer Security Association, and are the fastest spreading 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 left to spread. They can infect at different points during a file's use, for example, when it is opened, saved, closed, or deleted. HTML viruses use the scripts within HTML files to do their damage. These embedded scripts automatically come to life when the HTML page is viewed from a script-enabled browser.