A computer virus is a segment of code that has the ability to replicate. Viruses usually
replicate by infecting files. When a virus infects a file, it attaches a copy of itself
to the file in such a way that when the former is executed, the virus is also run.
When this happens, the infected file also becomes capable of infecting other files.
Generally, there are three kinds of viruses:
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FileFile viruses may come in different types– there are DOS viruses, Windows viruses, macro viruses, and script viruses. All of these share the same characteristics of viruses except that they infect different types of host files or programs.
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BootBoot viruses infect the partition table of hard disks and boot sector of hard disks and floppy disks.
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ScriptScript viruses are viruses 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 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 can be activated simply by double-clicking a *.vbs or *.js file from Windows Explorer.What is so special about script viruses? Unlike programming binary viruses, which require assembly-type programming knowledge, virus authors programs script viruses 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 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, 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. These types of viruses can infect
files belonging to different platforms (for example, Windows and Linux). However,
such viruses are very rare and seldom achieve 100% functionality.