A Virus is a self-replicating piece of code that attaches itself to other programs and usually requires human intervention to propagate. It affixes itself to other programs because a Virus cannot function as a standalone executable. Typically, it attaches some harmless code. The Virus carrier, or host, can be a standard executable, such as iTunesHelper.exe, or a data file that contains macro commands, such as Microsoft Word. An executable is a program that when opened causes the operating system to run the program. Microsoft Windows supports two types of executables, .COM files and .EXE files. Either can host a Virus. Some Viruses are benign or playful in intent and effect; some can be harmful, erasing data or causing the user's hard disk to require reformatting. There are three types of Viruses: System infectors infect executable code. Macro Viruses infect word processing applications and inject unwanted text. File infectors attach themselves to certain types of program files and then load along with the program. Viruses are usually sent as attachments to e-mails or in files to be downloaded. Viruses can also be present on a CD, and then transmitted when the user accesses the data on the disk. Viruses can make copies of themselves without human intervention. Although the user does not literally duplicate the code, he or she initiates the Virus by launching the host program to which the Virus is attached. Once active, the Virus can attach to any files or boot sectors in a computer to which the user is permitted to access. The three most common means by which Viruses are propagated are e-mail attachments, downloads and storing infected files in shared directories, where a document saved to a shared directory will likely be opened from other PCs by other users. The file servers for peer-to peer services, for example, are prime breeding grounds for Viruses. |