1. WHAT IS A VIRUS?
A Computer Virus is a computer program that can copy itself and infect a computer
A Computer Virus is a computer program that can copy itself and infect a computer. A true virus can only spread from one computer to another when its host is taken to the target computer. Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer. The first virus appeared in the early 1970’s, when it was used in a game played by some scientists. The target of this game was to fill the other person’s RAM memory using the viruses.

2. We can find Four different types of computer viruses
1- Trojan Horse
We can find four different types of computer viruses:
1-Trojan Horse: It facilitates unauthorized access to the user's computer. This kind of viruses are designed to allow a hacker remote access to a target computer system. Once a Trojan horse has been installed on a target computer system, it is possible for a hacker to access to it and do any kind of operation. This virus normally appears in software you can download, websites with executable stuff and sent files.
2-Computer Worm: It causes at least some harm to the network. It uses a network to send copies of itself to other computers in that network. Unlike a virus, it doesn’t have to be sent with an executable program.
2- Computer Worm

3. 3- Logic Bomb
3- Logic Bomb: will set off a malicious function when specified conditions are met. Usually viruses and worms contain logic bombs that start working when the hacker wants. The most famous logic bomb success happened in This logic bomb was responsible of the damages caused in the Trans-Siberian pipeline incident.
4-Hoax: Is all the false stuff that is use on the Internet in order to obtain valuable things from a user. This word is not only used talking about computers.
4- Hoax

4. HOW DO THEY WORK?
A file virus attaches itself to a file, usually an executable application. When click on it will start sending information to our computer until it gets infected.
Many viruses attach themselves to executable files that may be part of legitimate programs. If a user attempts to launch an infected program, the virus' code may be executed.

5. HOW ARE THEY TRANSFERRED?
These viruses spread normally via floppy disks which may come from virtually any source:  
- Unsolicited demonstration disks
- Brand-new software
- Disks used on your PC by salesmen or engineers
- Repaired hardware
A computer is infected with a boot sector virus if it is booted from an infected floppy disk. Boot sector infections cannot normally spread across a network. These viruses spread normally via floppy disks which may come from virtually any source:  
-Unsolicited demonstration disks
-Brand-new software
-Disks used on your PC by salesmen or engineers
-Repaired hardware

6. Unsolicited demonstration disks
Infected PC

7. PREVENTION MEASURES: Anti-Virus Software
Anti-Virus Software: Anti-Virus software is the best way to protect our computers from the viruses.
There are three different kind of anti-viral packages: activity monitors, authentication or change-detection software, and scanners. Each type has its own strengths and weaknesses.

8. Backup Data to an Unconnected Memory:
The second way of protecting our computers is Backup Data on an Unconnected Memory. This can be a nice way to protect it too, but we need an external device such as a hard drive or CD/DVD.
Backup Data to an Unconnected Memory: