Introduction:
A computer virus is a man-made harmful computer program that affects the normal functioning of a computer system. Computer viruses can transfer from different means to a computer without the knowledge and permission of the user. Computer viruses have the ability to replicate their source code and they can hide in other files. computer virus executed when such files are used for or executed.

Purpose of creating a virus:

1. To steal data and information
2. To earn money e
3. To expose programming ability
4. To destroy data information and files

Modes of virus transmission:

1. Sharing of infected external storage describe pen drive CD DVD
2. Using pirated software
3. Downloading the files from unknown websites
4. Opening virus-infected email messages
5. Exchanging of data and information over the network

Effects of computer virus:

1. Destroy data information and programs
2. Makes computer hang
3. Delete or remove files permanently
4. Reduce overall performance of computer
5. Spreads from one computer to another
6. Change the size of file

Symptoms of computer virus:

1. The program takes more time to load or fails to load
2. Irritating messages come on screen suddenly
3. Missing files or mismatch of files
4. Computer becomes slow
5. The computer gets shut down unexpectedly
6. The program opens automatically without giving instructions

1. Boot sector virus:-
   It is a startup infector virus that infects the bootstrap loader. The bootstrap loader and the file information are stored on the boot sector of a disk. When the computer is started this virus loads its source code first and takes control of the computer system. The boot sector virus affects the operating system of computers. Example: C- Brain, Disk Killer, etc
2. File infector virus or parasitic virus:-
   These viruses infect executable files or programs. The files having extensions such as .exe, .sys, .com are executable files or program files that are infected by file infector virus. Example: Ambulance virus, Die Heart, etc.
3. Multipartite virus:-
   This virus has the ability of file infector as well as boot sector virus. It can infect both the boot sector and executable files. For example Ghostball, Invader, etc.
4. Macro virus or document infector virus:-
   This virus infects specifically the document files created by Microsoft applications such as Word, Excel, PowerPoint, etc. So it is also called the document infector virus. Example: Love letter, Melissa, etc.
5. Script virus:-
   This virus infects script programming languages such as JavaScript, VB script. This virus can transfer through emails and by downloading files from web pages. Example: ILOVEYOU, HTML.Redlof. A, etc
6. WORMS:-
   It is a malicious computer program that replicates but does not hide itself in other files. It is similar to computer viruses and can cause negative effects too. Example: PSW Bugbear, Mapson, etc.
7. Trojan horse:-
   It is a destructive program that does not replicate, however, it can harm the computer system by providing remote access to someone else.

Types of computer viruses:

Antivirus:
An antivirus is a useful computer program that detects and removes viruses from the computer system. It is a utility program that protects computer hardware and software from being damaged due to viruses. Example: Quick Heal, Kaspersky, AVG(Anti Virus Guard), Norton, etc.

Advantages of antivirus:

1. Real-time protection of computer
2. Scan computer’s hard drive to ensure virus free
3. Detect computer virus
4. Scan portable and removable media like pen drive CD DVD etc

Methods of protection from computer viruses:

1. Install Powerful antivirus
2. Do not use pirated software
3. Update antivirus software timely
4. Do not download files from the internet and unknown sites.
5. Do not open unknown emails
6. Scan external media like pen drive CD DVD before using it
7. Use a strong password to protect from unauthorized access by others