1. BOOT Infectors

2. SYSTEM Infectors

3. GENERAL EXECUTABLE PROGRAM Infectors

They sometimes have the capability to trap soft booting(i.e. CTRL ALT DEL) and remain in control even if the system is booted from a non-infected floppy, thereby contaminating the clean floppy.

Boot infectors displaces information originally residing on the location, which they occupy. While writing onto the boot sector,the virus ensures that the boot record is not deleted. Once the virus is loaded, it automatically transfers control to the area where the boot record is available.

The reason behind doing this is that the boot record contains instructions to read IBMBIO.COM and IBMDOS.COM and if these files are not readable, access to the disk is not possible, and so the virus becomes ineffective.

Boot infectors typically create "Bad sectors". Boot infectors are the types, which once loaded would stay in the memory until the system is shut off, and until the disk reforms.

SYSTEM Infectors

This second category of viruses deals with components of the system itself. All machines without exception require an operating system in order to create an environment in which the operator works. In MS-DOS, COMMAND.COM contains all the internal commands.

If no such command file exists, commands such as COPY,DIR etc. are not loaded onto the memory when the machine is booted. The System Infectors attach themselves to a file such as COMMAND.COM or other memory resident files and manipulate these files.

System Infectors differ from Boot infectors in the sense that system infectors gain control after the computer is booted: and infects the hard disk or bootable floppies, which contain the appropriate system files only.

They have another peculiarity that they may activate after a given period of time or may instantly begin subtle modifications in system processing such as, increasing the time to perform system functions, scrambling of data, or modification of systems error messages or information messages.

GENERAL .COM OR .EXE Infectors

From the infection point of view, these viruses are most dangerous and devastating of the three classes of viruses. They attach themselves to program files and can spread to almost any executable program in any system. These viruses change the original program instructions into a "jump" to its own code and follows that code with a return to the original program.

As a result, whenever the program is executed, the virus gets loaded and executed first and then allows the original program to proceed. It remains memory resident and infects each and everyprogram that is loaded for execution.

By attaching themselves to EXE or COM files, they alter the file size and sometimes multiple infections renders program files too large to be accommodated in the memory.

By - TAIOC

About the author: Roger is a freelance writer. Article written for www.taioc.com