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## Viruses/Malware

A computer virus/malware is a segment of code that has the ability to replicate by infecting files. When a virus/malware infects a file, it attaches a copy of itself to the file in such a way that when the former executes, the virus/malware also runs. When this happens, the infected file also becomes capable of infecting other files. Like biological viruses, computer viruses/malware can spread quickly and are often difficult to eradicate.
In addition to replication, some computer viruses/malware share another commonality: a damage routine that delivers a payload. While payloads may only display messages or images, they can also destroy files, reformat your hard drive, or cause other damage. Even if the virus does not contain a damage routine, it can cause trouble by consuming storage space and memory, and degrading the overall performance of your computer.
Generally, there are three kinds of viruses/malware:

#### Types of Virus/Malware

Type | Description
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File | File viruses/malware may come in different types—there are DOS viruses/malware, Windows viruses/malware, macro viruses/malware, and script viruses/malware. All of these share the same characteristics of viruses/malware except that they infect different types of host files or programs.
Boot | Boot viruses/malware infect the partition table of hard disks and boot sector of hard disks and floppy disks.
Script | Script viruses/malware are viruses/malware written in script programming languages, such as Visual Basic Script and JavaScript and are usually embedded in HTML documents. VBScript (Visual Basic Script) and Jscript (JavaScript) viruses/malware make use of Microsoft's Windows Scripting Host to activate themselves and infect other files. Since Windows Scripting Host is available on Windows 98, Windows 2000 and other Windows operating systems, the viruses/malware can be activated simply by double-clicking a `*.vbs` or `*.js` file from Windows Explorer. What is so special about script viruses/malware? Unlike programming binary viruses/malware, which requires assembly-type programming knowledge, virus/malware authors program script viruses/malware as text. A script virus can achieve functionality without low-level programming and with code as compact as possible. It can also use predefined objects in Windows to make accessing many parts of the infected system easier (for example, for file infection, for mass-mailing). Furthermore, since the code is text, it is easy for others to read and imitate the coding paradigm. Because of this, many script viruses/malware have several modified variants. For example, shortly after the "I love you" virus appeared, antivirus vendors found modified copies of the original code, which spread themselves with different subject lines, or message bodies.

Whatever their type is, the basic mechanism remains the same. A virus contains code that explicitly copies itself. In the case of file viruses/malware, this usually entails making modifications to gain control when a user accidentally executes the infected program. After the virus code has finished execution, in most cases, it passes back the control to the original host program to give the user an impression that nothing is wrong with the infected file.
Take note that there are also cross-platform viruses/malware. These types of viruses/malware can infect files belonging to different platforms (for example, Windows and Linux). However, such viruses/malware are very rare and seldom achieve 100% functionality.
Related information

- Virus/Malware Writers
- Malware Naming