Viruses are classified on the basic of their mode of existence and there are three categories of viruses.
1. BOOT Infector:-
2. SYSTEM Infector:
3. GENERAL EXECUTABLE PROGRAM Infectors:-
As the name suggests, they are characterized by the fact that they are physically reside in the boot sector  sector of the disk. A system infected by such a virus will have the virus residing in a particular area of the disk rather then in a program file. These viruses get loaded soon after the Power Of Self Test and control the system and remains in control at all times. They some times have the capacity to trap soft booting (CTRL ALT DEL) and remain in control even if the system is booted from a non infected floppy, thereby contaminating the clean floppy.
The second category of virus deals with components of the system itself. All machines without execution 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 into 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.
GENERAL EXECUTABLE PROGRAM Infectors:-
From the infection point of view, these 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 follow 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 process. It remains memory resident and infects each and every program that is loaded for execution.
Types of Viruses:-
A worm is software that uses computer networks and security holes to replicate, or affect other programs. A copy of the worm passes through the network to another machine that has a specific security hole. It then copies itself to the new machine, and then starts replicating from there.
Macro viruses’ use commands-called macros-embedded in the other software applications to infect and spread to other files used by that software. For example, Microsoft Word and Excel have macros, and macro viruses can spread by exploiting these commands.
Boot Sector viruses:-
Boot sector viruses are older types of virus and are not so common today. They used to infect computers startup programs, so that the virus would become active as soon as the computer started up.
Trojan Horses are programs that claim to perform a particular function but which in fact do something different. For example, they could infect your computer with a virus or erase your files.
Backdoor Trojans are programs that allow other computer users to remotely control your computer via a local area network or the internet.
These types of virus hides permanently in the RAM memory. From here it can control and intercept all of the operations carried out by the system corrupting files and programs that are opened, closed, copied etc.
Direct Action Viruses:-
When a specific condition is met (such as the execution of the virus) these virus will go into action and infect files in the directory or folder that it in and others as well.
These types of viruses are characterized by the fact that it deletes the information contained in the files that it infects, rendering them particularly or totally useless once they have been infected.
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