Computer Virus is a malicious software program written intentionally to enter a computer without the user's permission or knowledge. It has the ability to replicate itself, thus continues to spread. Some viruses do little but replicate, while others can cause severe harm or adversely affect program and performance of the system. A virus should never be assumed harmless and left on a system.
There are different types of computer viruses which can be classified according to their origin, techniques, types of files they infect, where they hide, the kind of damage they cause, the type of operating system or platform they attack etc. Let us have a look at few of them.
Resident Virus
This type of virus is a permanent as it dwells in the RAM. From there it can overcome and interrupt all the operations executed by the system. It can corrupt files and programs that are opened, closed, copied, renamed etc.
Examples: Randex, CMJ, Meve, and MrKlunky.
Direct Action Viruses
The main purpose of this virus is to replicate and take action when it is executed. When a specific condition is met, the virus will go into action and infect files in the directory or folder that it is in as well as directories that are specified in the AUTOEXEC.BAT file path. This batch file is always located in the root directory of the hard disk and carries out certain operations when the computer is booted.
Examples: Vienna virus.
Overwrite Viruses
Virus of this kind is characterized by the fact that it deletes the information contained in the files that it infects, rendering them partially or totally useless once they have been infected. The only way to clean a file infected by an overwrite virus is to delete the file completely, thus losing the original content.
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