What viruses do When executed, viruses go through two phases. First, they infect other programs on your disk. This may or may not happen immediately, as some viruses remain resident in memory until triggered. Next, the virus attacks, perhaps deleting files, corrupting data or generally affecting your PC's functionality. This type of virus can render your hard disk unreadable.
File viruses look for executable files and overwrite part of the program.
When the program is executed, the virus code also executes and infects more files. Although the program is infected, it usually appears to work properly and the virus may go undetected for some time. Macro viruses use an application's own macro programming language to distribute themselves. Upon executing the macro, the virus infects your document, rather than the program itself. These viruses are particularly contagious as they can be transmitted through e-mail as attached documents.